| **Deposits (Deposit Composition) (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                  | Dec. 31, 2013                                                                  |
|----------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------|--------------------------------------------------------------------------------|
| **Deposits, by Component, Alternative [Abstract]**                                           |                                                                           |                                                                           |
| Noninterest-bearing demand deposits                                                          | $ 323,776us-gaap_NoninterestBearingDomesticDepositDemand                       | $ 290,461us-gaap_NoninterestBearingDomesticDepositDemand                       |
| Interest-bearing demand deposits                                                             | 358,162us-gaap_InterestBearingDomesticDepositDemand                            | 347,791us-gaap_InterestBearingDomesticDepositDemand                            |
| Savings deposits                                                                             | 86,349us-gaap_InterestBearingDomesticDepositSavings                            | 80,507us-gaap_InterestBearingDomesticDepositSavings                            |
| Money market deposits                                                                        | 437,821us-gaap_InterestBearingDomesticDepositMoneyMarket                       | 447,672us-gaap_InterestBearingDomesticDepositMoneyMarket                       |
| Brokered deposits                                                                            | 37,673us-gaap_InterestBearingDomesticDepositBrokered                           | 29,219us-gaap_InterestBearingDomesticDepositBrokered                           |
| Time deposits less than $250,000                                                             | 500,672us-gaap_TimeDepositsLessThan100000                                      | 517,687us-gaap_TimeDepositsLessThan100000                                      |
| Time deposits $250,000 or more                                                               | 49,967us-gaap_TimeDeposits100000OrMoreDomestic                                 | 35,368us-gaap_TimeDeposits100000OrMoreDomestic                                 |
| Total deposits                                                                               | $ 1,794,420us-gaap_Deposits                                                    | $ 1,748,705us-gaap_Deposits                                                    |
| Noninterest-bearing demand deposits, Percentage                                              | 18.04%cob_PercentageOfNoninterestBearingDomesticDepositsToDepositsDemand       | 16.61%cob_PercentageOfNoninterestBearingDomesticDepositsToDepositsDemand       |
| Interest-bearing demand deposits, Percentage                                                 | 19.96%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsDemand      | 19.89%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsDemand      |
| Savings deposits, Percentage                                                                 | 4.81%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsSavings      | 4.61%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsSavings      |
| Money market deposits, Percentage                                                            | 24.40%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsMoneyMarket | 25.60%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsMoneyMarket |
| Brokered deposits, Percentage                                                                | 2.10%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsBrokered     | 1.67%us-gaap_PercentageOfInterestBearingDomesticDepositsToDepositsBrokered     |
| Time deposits less than $250,000, Percentage                                                 | 27.90%cob_PercentageOfTimeDepositsLessThan100000ToDeposits                     | 29.60%cob_PercentageOfTimeDepositsLessThan100000ToDeposits                     |
| Time deposits $250,000 or more, Percentage                                                   | 2.79%cob_PercentageOfTimeDepositsMoreThan100000ToDeposits                      | 2.02%cob_PercentageOfTimeDepositsMoreThan100000ToDeposits                      |
| Total deposits, Percentage                                                                   | 100.00%cob_PercentageOfDeposits                                                | 100.00%cob_PercentageOfDeposits                                                |

| X                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Deposits
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Noninterest-bearing Domestic Deposits to Deposits, Demand
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Time Deposits, Less than 100000 to Deposits
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Time Deposits, More than 100000 to Deposits
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.
+ 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 210.9-03.12) -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 12 -Article 9
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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest-bearing domestic third-party deposits placed by or through the assistance of a deposit broker, also known as brokered deposit liabilities.
+ 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 210.9-03.12) -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 12 -Article 9
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of money in interest-bearing domestic accounts that entitle the depositor to withdraw funds at any time without prior notice, also known as demand deposits.
+ 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 210.9-03.12) -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 12 -Article 9
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest-bearing domestic money market demand account deposit liabilities.
+ 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 210.9-03.12) -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 12 -Article 9
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest-bearing domestic savings deposit liabilities. Saving deposits bear interest and have no stated maturity.
+ 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 210.9-03.12) -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 12 -Article 9
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of money in noninterest-bearing domestic accounts that entitle the depositor to withdraw funds at any time without prior notice, also known as demand deposits.
+ 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 210.9-03.12) -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 12 -Article 9
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| X                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of interest-bearing domestic brokered deposit liabilities to total deposit liabilities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of interest-bearing domestic demand deposit liabilities to total deposit liabilities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of interest-bearing domestic money market demand account deposit liabilities to total deposit liabilities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of interest-bearing domestic savings deposit liabilities to total deposit liabilities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate amount of domestic time deposits (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $100,000 or more.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6479006&loc=d3e64164-112818
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| X                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate amount of domestic or foreign time deposits (including certificates of deposits, individual retirement accounts and open accounts) that total to less than $100,000.
+ References
No definition available.
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