| **Loans and Allowance for Loan Losses (Granite Purchased Loans) (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                                                                 | Dec. 31, 2013                                                                                                                                                                                                                                                  | Dec. 31, 2012                                                                                                                                                             | Dec. 31, 2011                                     |
|-----------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------|
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Allowance of loan losses                                                                                                    | $ 20,345us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                                                             | $ 26,785us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                                                              | $ 29,314us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                         | $ 39,360us-gaap_LoansAndLeasesReceivableAllowance |
| Loans held for investment                                                                                                   | 1,357,788us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                                                                  | 1,212,248us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                                                                   |                                                                                                                                                                      |                                              |
| Granite Corp. [Member]                                                                                                      |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Purchased impaired                                                                                                          | 122,842cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                     | 161,652cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                      | 228,392cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember |                                              |
| Purchased contractual                                                                                                       | 25,948cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                                                                    | 28,448cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                                                                     |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 148,790us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                                                                            | 190,100us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                                                                             |                                                                                                                                                                      |                                              |
| Unpaid principal balance                                                                                                    | 153,774cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                                                                             | 202,174cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember                                                                                                                                              |                                                                                                                                                                      |                                              |
| Granite Corp. [Member] | Commercial and agricultural [Member]                                                               |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Purchased impaired                                                                                                          | 7,535cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember | 10,729cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember |                                                                                                                                                                      |                                              |
| Purchased contractual                                                                                                       | 4,288cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember                                               | 5,948cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember                                                |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 11,823us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember                                                       | 16,677us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember                                                        |                                                                                                                                                                      |                                              |
| Unpaid principal balance                                                                                                    | 10,508cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember                                                        | 16,452cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialAndAgriculturalPortfolioSegmentMember                                                         |                                                                                                                                                                      |                                              |
| Granite Corp. [Member] | Real estate - construction [Member]                                                                |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Purchased impaired                                                                                                          | 9,197cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                     | 9,893cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                      |                                                                                                                                                                      |                                              |
| Purchased contractual                                                                                                       | 0cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                                                                       | 0cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                                                                        |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 9,197us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                                                                            | 9,893us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                                                                             |                                                                                                                                                                      |                                              |
| Unpaid principal balance                                                                                                    | 10,463cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                                                                            | 11,368cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConstructionLoansMember                                                                             |                                                                                                                                                                      |                                              |
| Granite Corp. [Member] | Real estate - mortgage, 1-4 family residential [Member]                                            |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Purchased impaired                                                                                                          | 17,737cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                  | 26,853cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                   |                                                                                                                                                                      |                                              |
| Purchased contractual                                                                                                       | 21,660cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                                                                | 22,127cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                                                                 |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 39,397us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                                                                         | 48,980us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                                                                          |                                                                                                                                                                      |                                              |
| Unpaid principal balance                                                                                                    | 41,295cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                                                                          | 51,359cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember                                                                           |                                                                                                                                                                      |                                              |
| Granite Corp. [Member] | Real estate - mortgage, commercial [Member]                                                        |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Purchased impaired                                                                                                          | 87,345cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                  | 113,178cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                  |                                                                                                                                                                      |                                              |
| Purchased contractual                                                                                                       | 0cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                                                                     | 373cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                                                                    |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 87,345us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                                                                         | 113,551us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                                                                         |                                                                                                                                                                      |                                              |
| Unpaid principal balance                                                                                                    | 90,830cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                                                                          | 122,197cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= cob_CommercialMortgageLoansMember                                                                          |                                                                                                                                                                      |                                              |
| Granite Corp. [Member] | Consumer [Member]                                                                                  |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Purchased impaired                                                                                                          | 1,028cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                          | 999cob_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountBeforeProvisionForAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                             |                                                                                                                                                                      |                                              |
| Purchased contractual                                                                                                       | 0cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                                                                            | 0cob_LoansAndLeasesReceivablesAcquiredLoansContractualBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                                                                             |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 1,028us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                                                                                 | 999us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                                                                                    |                                                                                                                                                                      |                                              |
| Unpaid principal balance                                                                                                    | 678cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                                                                                    | 798cob_LoansAndLeasesRecievableUnpaidPrincipalBalance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ConsumerLoanMember                                                                                     |                                                                                                                                                                      |                                              |
| PI [Member]                                                                                                                 |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 122,842us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedImpairedLoansMember                                                                                                                            | 161,652us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedImpairedLoansMember                                                                                                                             |                                                                                                                                                                      |                                              |
| PI [Member] | Granite Corp. [Member]                                                                                        |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Allowance of loan losses                                                                                                    | 3,200us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedImpairedLoansMember                                                                                | 5,600us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedImpairedLoansMember                                                                                 |                                                                                                                                                                      |                                              |
| PC [Member]                                                                                                                 |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Loans held for investment                                                                                                   | 1,234,946us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedContractsAndOriginatedLoansMember                                                                                                            | 1,050,596us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedContractsAndOriginatedLoansMember                                                                                                             |                                                                                                                                                                      |                                              |
| PC [Member] | Granite Corp. [Member]                                                                                        |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| **Financing Receivable [Line Items]**                                                                                       |                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                           |                                                                                                                                                                      |                                              |
| Allowance of loan losses                                                                                                    | $ 300us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedContractsAndOriginatedLoansMember                                                                  | $ 300us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_InternalCreditAssessmentAxis= cob_PurchasedContractsAndOriginatedLoansMember                                                                   |                                                                                                                                                                      |                                              |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The recorded balance before provision for allowance.  This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans and leases held in portfolio acquired with no evidence of credit deterioration, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of loans and leases receivables with related allowance for credit losses and without related allowance for credit losses.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesRecievableUnpaidPrincipalBalance**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                            |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section L -Subsection 1
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522
+ Details**Name:**us-gaap_LoansAndLeasesReceivableGrossCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=cob_PurchasedContractsAndOriginatedLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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