| **Investment Securities (Continuous Unrealized Loss Position) (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                  | Dec. 31, 2014                                                                                                                                                                                                                                  |
|---------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | $ 135,012us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                       | $ 1,154us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                         |
| Less than 12 months, gross unrealized losses                                                                              | 657cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss                                                                                                                                               | 4cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss                                                                                                                                                 |
| 12 months or more, estimated fair value                                                                                   | 145,265us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                         | 277,630us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                         |
| 12 months or more, gross unrealized losses                                                                                | 3,245cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss                                                                                                                                             | 6,162cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss                                                                                                                                             |
| Total, estimated fair value                                                                                               | 280,277us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                             | 278,784us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                             |
| Total, gross unrealized losses                                                                                            | 3,902cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                             | 6,166cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                             |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 34,426us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                            | 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                                 |
| Less than 12 months, gross unrealized losses                                                                              | 80cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss                                                                                                                                                  | 0cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss                                                                                                                                                   |
| 12 months or more, estimated fair value                                                                                   | 39,603us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                            | 122,876us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                           |
| 12 months or more, gross unrealized losses                                                                                | 292cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss                                                                                                                                                 | 1,702cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss                                                                                                                                               |
| Total, estimated fair value                                                                                               | 74,029us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                                | 122,876us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                               |
| Total, gross unrealized losses                                                                                            | 372cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                                 | 1,702cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                               |
| **Continuous Unrealized Loss Position [Abstract]**                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 169,438cob_ContinuousUnrealizedLossPositionLessthanTwelveMonthsFairValue                                                                                                                                                                       | 1,154cob_ContinuousUnrealizedLossPositionLessthanTwelveMonthsFairValue                                                                                                                                                                         |
| Less than 12 months, gross unrealized losses                                                                              | 737cob_ContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses                                                                                                                                                                         | 4cob_ContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses                                                                                                                                                                           |
| 12 months or more, estimated fair value                                                                                   | 184,868cob_ContinuousUnrealizedLossPositionTwelveMonthsorLongerFairValue                                                                                                                                                                       | 400,506cob_ContinuousUnrealizedLossPositionTwelveMonthsorLongerFairValue                                                                                                                                                                       |
| 12 months or more, gross unrealized losses                                                                                | 3,537cob_ContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses                                                                                                                                                                       | 7,864cob_ContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses                                                                                                                                                                       |
| Total, estimated fair value                                                                                               | 354,306cob_ContinuousUnrealizedLossPositionFairValue                                                                                                                                                                                           | 401,660cob_ContinuousUnrealizedLossPositionFairValue                                                                                                                                                                                           |
| Total, gross unrealized losses                                                                                            | 4,274cob_ContinuousUnrealizedLossPositionAggregateLosses                                                                                                                                                                                       | 7,868cob_ContinuousUnrealizedLossPositionAggregateLosses                                                                                                                                                                                       |
| Residential mortgage-backed securities - GSE [Member]                                                                     |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 80,677us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember  | 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember       |
| Less than 12 months, gross unrealized losses                                                                              | 557cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember       | 0cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember         |
| 12 months or more, estimated fair value                                                                                   | 145,265us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember | 245,457us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember |
| 12 months or more, gross unrealized losses                                                                                | 3,245cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember     | 5,593cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember     |
| Total, estimated fair value                                                                                               | 225,942us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                     | 245,457us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                     |
| Total, gross unrealized losses                                                                                            | 3,802cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                     | 5,593cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                     |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 34,426us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember    | 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember         |
| Less than 12 months, gross unrealized losses                                                                              | 80cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember          | 0cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember           |
| 12 months or more, estimated fair value                                                                                   | 39,603us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember    | 112,878us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember   |
| 12 months or more, gross unrealized losses                                                                                | 292cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember         | 1,635cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember       |
| Total, estimated fair value                                                                                               | 74,029us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                        | 112,878us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                       |
| Total, gross unrealized losses                                                                                            | 372cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                         | 1,635cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember                       |
| Residential mortgage-backed securities - Private [Member]                                                                 |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 1,135us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                 | 1,154us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                 |
| Less than 12 months, gross unrealized losses                                                                              | 4cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                       | 4cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                       |
| 12 months or more, estimated fair value                                                                                   | 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                     | 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                     |
| 12 months or more, gross unrealized losses                                                                                | 0cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                       | 0cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                       |
| Total, estimated fair value                                                                                               | 1,135us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                     | 1,154us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                     |
| Total, gross unrealized losses                                                                                            | 4cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                       | 4cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                       |
| Commercial mortgage-backed securities - GSE [Member]                                                                      |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 22,208us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                        | 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                             |
| Less than 12 months, gross unrealized losses                                                                              | 71cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                              | 0cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                               |
| 12 months or more, estimated fair value                                                                                   | 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                             | 21,958us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                        |
| 12 months or more, gross unrealized losses                                                                                | 0cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                               | 419cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                             |
| Total, estimated fair value                                                                                               | 22,208us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                            | 21,958us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                            |
| Total, gross unrealized losses                                                                                            | 71cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                              | 419cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesGseMember                                                             |
| Commercial mortgage-backed securities - Private [Member]                                                                  |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 |                                                                                                                                                                                                                                           | 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                      |
| Less than 12 months, gross unrealized losses                                                                              |                                                                                                                                                                                                                                           | 0cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                        |
| 12 months or more, estimated fair value                                                                                   |                                                                                                                                                                                                                                           | 10,215us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                 |
| 12 months or more, gross unrealized losses                                                                                |                                                                                                                                                                                                                                           | 150cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                      |
| Total, estimated fair value                                                                                               |                                                                                                                                                                                                                                           | 10,215us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                     |
| Total, gross unrealized losses                                                                                            |                                                                                                                                                                                                                                           | 150cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                      |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 |                                                                                                                                                                                                                                           | 0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                        |
| Less than 12 months, gross unrealized losses                                                                              |                                                                                                                                                                                                                                           | 0cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                          |
| 12 months or more, estimated fair value                                                                                   |                                                                                                                                                                                                                                           | 9,998us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                    |
| 12 months or more, gross unrealized losses                                                                                |                                                                                                                                                                                                                                           | 67cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                         |
| Total, estimated fair value                                                                                               |                                                                                                                                                                                                                                           | 9,998us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                        |
| Total, gross unrealized losses                                                                                            |                                                                                                                                                                                                                                           | 67cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                         |
| Corporate notes [Member]                                                                                                  |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Less than 12 months, estimated fair value                                                                                 | 30,992us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                  |                                                                                                                                                                                                                                           |
| Less than 12 months, gross unrealized losses                                                                              | 25cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                        |                                                                                                                                                                                                                                           |
| Total, estimated fair value                                                                                               | 30,992us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                      |                                                                                                                                                                                                                                           |
| Total, gross unrealized losses                                                                                            | $ 25cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateNoteSecuritiesMember                                                                      |                                                                                                                                                                                                                                           |

| X                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
+ References
No definition available.
+ Details**Name:**cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
+ References
No definition available.
+ Details**Name:**cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss
+ References
No definition available.
+ Details**Name:**cob_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses
+ References
No definition available.
+ Details**Name:**cob_ContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cob_ContinuousUnrealizedLossPositionAbstract**Namespace Prefix:**cob_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Aggregate Losses
+ References
No definition available.
+ Details**Name:**cob_ContinuousUnrealizedLossPositionAggregateLosses**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Fair Value
+ References
No definition available.
+ Details**Name:**cob_ContinuousUnrealizedLossPositionFairValue**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses
+ References
No definition available.
+ Details**Name:**cob_ContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value
+ References
No definition available.
+ Details**Name:**cob_ContinuousUnrealizedLossPositionLessthanTwelveMonthsFairValue**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value
+ References
No definition available.
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| - Definition
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
+ References
No definition available.
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| - Definition
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss
+ References
No definition available.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss
+ References
No definition available.
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| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
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| - Definition
This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
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