| **Fair Values of Assets and Liabilities (Measured on Recurring Basis) (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                                                                                                                                                      | Dec. 31, 2014                                                                                                                                                                                                                                                                                                                                                                      |
|-----------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | $ 367,842us-gaap_AvailableForSaleSecurities                                                                                                                                                                                                                                                                                                                                        | $ 350,040us-gaap_AvailableForSaleSecurities                                                                                                                                                                                                                                                                                                                                        | [1] |
| US Government-sponsored Enterprises Debt Securities [Member]                                                                      |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 2,036us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                                                                                                                                                                                             | 2,044us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                                                                                                                                                                                             |  |
| Residential mortgage-backed securities - GSE [Member]                                                                             |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 285,719us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                                                                                                                                                                                                  | 290,145us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                                                                                                                                                                                                  |  |
| Residential mortgage-backed securities - Private [Member]                                                                         |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 16,430us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                                                                                                                                                                                                 | 17,271us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                                                                                                                                                                                                 |  |
| Commercial mortgage-backed securities - GSE [Member]                                                                              |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 22,208us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                                                                                                                                                                                                         | 21,958us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                                                                                                                                                                                                         |  |
| Commercial mortgage-backed securities - Private [Member]                                                                          |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 10,457us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                                                                                                                                                                                                  | 10,215us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                                                                                                                                                                                                  |  |
| Corporate notes [Member]                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 30,992us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                                                                                                                                                                                                   | 8,407us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                                                                                                                                                                                                    |  |
| Level 1 [Member]                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                                                                                                                                                           | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                                                                                                                                                           |  |
| Level 2 [Member]                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 367,842us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                                                                                     | 350,040us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                                                                                     |  |
| Level 3 [Member]                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                                                                                           | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                                                                                           |  |
| Recurring [Member] | Total [Member]                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 367,842us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                         | 350,040us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                         |  |
| Mortgage servicing rights                                                                                                         | 1,896us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                      | 1,726us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                      |  |
| Total assets at fair value                                                                                                        | 368,693us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                          | 351,241us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                          |  |
| Recurring [Member] | Total [Member] | US Government-sponsored Enterprises Debt Securities [Member]                                |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 2,036us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                            | 2,044us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                            |  |
| Recurring [Member] | Total [Member] | Residential mortgage-backed securities - GSE [Member]                                       |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 285,719us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember | 290,145us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember |  |
| Recurring [Member] | Total [Member] | Residential mortgage-backed securities - Private [Member]                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 16,430us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                | 17,271us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                |  |
| Recurring [Member] | Total [Member] | Commercial mortgage-backed securities - GSE [Member]                                        |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 22,208us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                        | 21,958us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                        |  |
| Recurring [Member] | Total [Member] | Commercial mortgage-backed securities - Private [Member]                                    |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 10,457us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                 | 10,215us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                 |  |
| Recurring [Member] | Total [Member] | Corporate notes [Member]                                                                    |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 30,992us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                  | 8,407us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                   |  |
| Recurring [Member] | Level 1 [Member]                                                                                             |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                                | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                                |  |
| Mortgage servicing rights                                                                                                         | 0us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                           | 0us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                           |  |
| Total assets at fair value                                                                                                        | 0us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                                 | 0us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                                 |  |
| Recurring [Member] | Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member]                              |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                 | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                 |  |
| Recurring [Member] | Level 1 [Member] | Residential mortgage-backed securities - GSE [Member]                                     |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                        | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                        |  |
| Recurring [Member] | Level 1 [Member] | Residential mortgage-backed securities - Private [Member]                                 |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                      | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                      |  |
| Recurring [Member] | Level 1 [Member] | Commercial mortgage-backed securities - GSE [Member]                                      |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          |                                                                                                                                                                                                                                                                                                                                                                               | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                              |  |
| Recurring [Member] | Level 1 [Member] | Commercial mortgage-backed securities - Private [Member]                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                       | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                       |  |
| Recurring [Member] | Level 1 [Member] | Corporate notes [Member]                                                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                        | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                        |  |
| Recurring [Member] | Level 2 [Member]                                                                                             |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 367,842us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                          | 350,040us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                          |  |
| Mortgage servicing rights                                                                                                         | 0us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                           | 0us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                           |  |
| Total assets at fair value                                                                                                        | 366,797us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                           | 349,515us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                           |  |
| Recurring [Member] | Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]                              |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 2,036us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                             | 2,044us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                             |  |
| Recurring [Member] | Level 2 [Member] | Residential mortgage-backed securities - GSE [Member]                                     |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 285,719us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                  | 290,145us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                  |  |
| Recurring [Member] | Level 2 [Member] | Residential mortgage-backed securities - Private [Member]                                 |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 16,430us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                 | 17,271us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                 |  |
| Recurring [Member] | Level 2 [Member] | Commercial mortgage-backed securities - GSE [Member]                                      |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 22,208us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                         | 21,958us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                         |  |
| Recurring [Member] | Level 2 [Member] | Commercial mortgage-backed securities - Private [Member]                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 10,457us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                  | 10,215us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                  |  |
| Recurring [Member] | Level 2 [Member] | Corporate notes [Member]                                                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 30,992us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                   | 8,407us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                    |  |
| Recurring [Member] | Level 3 [Member]                                                                                             |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                                | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                                |  |
| Mortgage servicing rights                                                                                                         | 1,896us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                       | 1,726us-gaap_ServicingAssetAtFairValueAmount/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                       |  |
| Total assets at fair value                                                                                                        | 1,896us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                             | 1,726us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                                                                             |  |
| Recurring [Member] | Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member]                              |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                 | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                 |  |
| Recurring [Member] | Level 3 [Member] | Residential mortgage-backed securities - GSE [Member]                                     |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                        | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                        |  |
| Recurring [Member] | Level 3 [Member] | Residential mortgage-backed securities - Private [Member]                                 |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                      | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                      |  |
| Recurring [Member] | Level 3 [Member] | Commercial mortgage-backed securities - GSE [Member]                                      |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          |                                                                                                                                                                                                                                                                                                                                                                               | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                              |  |
| Recurring [Member] | Level 3 [Member] | Commercial mortgage-backed securities - Private [Member]                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                       | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                       |  |
| Recurring [Member] | Level 3 [Member] | Corporate notes [Member]                                                                  |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Available-for-sale investment securities                                                                                          | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                        | 0us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                        |  |
| Interest Rate Swap [Member] | Level 1 [Member]                                                                                    |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | 0us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Interest Rate Swap [Member] | Level 2 [Member]                                                                                    |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | (1,044)us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Interest Rate Swap [Member] | Level 3 [Member]                                                                                    |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | 0us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Interest Rate Swap [Member] | Recurring [Member] | Total [Member]                                                                 |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | (1,045)us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                             | (525)us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                               |  |
| Interest Rate Swap [Member] | Recurring [Member] | Level 1 [Member]                                                               |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | 0us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                    | 0us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                    |  |
| Interest Rate Swap [Member] | Recurring [Member] | Level 2 [Member]                                                               |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | (1,045)us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                              | (525)us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                |  |
| Interest Rate Swap [Member] | Recurring [Member] | Level 3 [Member]                                                               |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| **Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]**                                                   |                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                               |  |
| Derivative assets                                                                                                                 | $ 0us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                  | $ 0us-gaap_DerivativeAssets/ us-gaap_DerivativeInstrumentRiskAxis= us-gaap_InterestRateSwapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                                                  |  |
|                                                                                                                                   |
| [1]Derived from audited consolidated financial statements                                                                         |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.
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| - Definition
Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.
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