| **Investment Securities (Other Disclosures) (Details) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2015securities                                                                                           | Dec. 31, 2014securities                                                                              |
|--------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------|
| **Schedule of Available-for-sale Securities [Line Items]**                                             |                                                                                                              |                                                                                                 |
| FHLB stock                                                                                             | $ 4.9us-gaap_FederalHomeLoanBankStock                                                                             | $ 4.9us-gaap_FederalHomeLoanBankStock                                                                |
| FRBR stock                                                                                             | 9.6us-gaap_FederalReserveBankStock                                                                                | 4.7us-gaap_FederalReserveBankStock                                                                   |
| Investment securities portfolio available as lendable collateral                                       | 200.8cob_AvailableForSaleSecuritiesAvailableCollateral                                                            |                                                                                                 |
| Available-for-sale securities in an unrealized loss position                                           | 15us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1              | 33us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 |
| Deposits [Member]                                                                                      |                                                                                                              |                                                                                                 |
| **Schedule of Available-for-sale Securities [Line Items]**                                             |                                                                                                              |                                                                                                 |
| Investment securities portfolio pledged as collateral                                                  | 120.2us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral/ cob_CollateralAxis= us-gaap_DepositsMember            |                                                                                                 |
| Retail repurchase agreements [Member]                                                                  |                                                                                                              |                                                                                                 |
| **Schedule of Available-for-sale Securities [Line Items]**                                             |                                                                                                              |                                                                                                 |
| Investment securities portfolio pledged as collateral                                                  | 16.5us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral/ cob_CollateralAxis= us-gaap_RepurchaseAgreementsMember |                                                                                                 |
| Others [Member]                                                                                        |                                                                                                              |                                                                                                 |
| **Schedule of Available-for-sale Securities [Line Items]**                                             |                                                                                                              |                                                                                                 |
| Investment securities portfolio pledged as collateral                                                  | $ 170.9us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral/ cob_CollateralAxis= cob_OthersMember                |                                                                                                 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This represents the amount of availalbe-for-sale securities that are not currently pledged to any debts, agreements or any other arrangements and available as lendable collateral.
+ References
No definition available.
+ Details**Name:**cob_AvailableForSaleSecuritiesAvailableCollateral**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.
+ 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 (b)(3) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The fair value of available-for-sale securities that serve as collateral for borrowings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5086-111524
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478792&loc=d3e63345-112809
+ Details**Name:**us-gaap_FederalHomeLoanBankStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478792&loc=d3e63345-112809
+ Details**Name:**us-gaap_FederalReserveBankStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cob_CollateralAxis=cob_OthersMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cob_CollateralAxis=us-gaap_DepositsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cob_CollateralAxis=us-gaap_RepurchaseAgreementsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |