| **Investment Securities (Other Disclosures) (Details) (USD $)**  | Dec. 31, 2013 | Dec. 31, 2012 |
|------------------------------------------------------------------|---------------|---------------|
| **Schedule of Available-for-sale Securities [Line Items]**       |          |          |
| FHLB stock                                                       | $ 5,900,000   | $ 6,300,000   |
| FRBR stock                                                       | 4,300,000     | 3,100,000     |
| Investment securities portfolio available as lendable collateral | 376,300,000   | 447,300,000   |
| Deposits [Member]                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**       |          |          |
| Investment securities portfolio pledged as collateral            | 91,200,000    | 111,400,000   |
| Retail repurchase agreements [Member]                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**       |          |          |
| Investment securities portfolio pledged as collateral            | 18,800,000    |          |
| Others [Member]                                                  |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**       |          |          |
| Investment securities portfolio pledged as collateral            | $ 77,400,000  | $ 6,100,000   |

| 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
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 |