| **Investment Securities (Unrealized Gain/Loss Positions) (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2013 | Dec. 31, 2012 |
|----------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | $ 438,057     | $ 558,818     |
| Gross unrealized gains                                                                                               | 3,048         | 7,451         |
| Gross unrealized losses                                                                                              | 26,491        | 1,419         |
| Estimated fair value                                                                                                 | 414,614       | 564,850       |
| **Held-to-maturity Securities [Abstract]**                                                                           |          |          |
| Amortized cost                                                                                                       | 151,795       |          |
| Gross unrealized gains                                                                                               | 0             |          |
| Gross unrealized losses                                                                                              | 10,670        |          |
| Estimated fair value                                                                                                 | 141,125       |          |
| Amortized cost, Total                                                                                                | 589,852       |          |
| Gross unrealized gains, Total                                                                                        | 3,048         |          |
| Gross unrealized losses, Total                                                                                       | 37,161        |          |
| Estimated fair value, Total                                                                                          | 555,739       |          |
| Obligations of U.S. Treasury and government agencies [Member]                                                        |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       |          | 6,646         |
| Gross unrealized gains                                                                                               |          | 335           |
| Gross unrealized losses                                                                                              |          | 0             |
| Estimated fair value                                                                                                 |          | 6,981         |
| Obligations of U.S. government sponsored enterprises [Member]                                                        |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 2,051         | 22,118        |
| Gross unrealized gains                                                                                               | 26            | 55            |
| Gross unrealized losses                                                                                              | 0             | 0             |
| Estimated fair value                                                                                                 | 2,077         | 22,173        |
| Obligations of states and political subdivisions [Member]                                                            |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       |          | 5,918         |
| Gross unrealized gains                                                                                               |          | 120           |
| Gross unrealized losses                                                                                              |          | 0             |
| Estimated fair value                                                                                                 |          | 6,038         |
| Residential mortgage-backed securities - GSE [Member]                                                                |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 364,513       | 436,344       |
| Gross unrealized gains                                                                                               | 974           | 5,678         |
| Gross unrealized losses                                                                                              | 24,340        | 948           |
| Estimated fair value                                                                                                 | 341,147       | 441,074       |
| **Held-to-maturity Securities [Abstract]**                                                                           |          |          |
| Amortized cost                                                                                                       | 141,724       |          |
| Gross unrealized gains                                                                                               | 0             |          |
| Gross unrealized losses                                                                                              | 9,907         |          |
| Estimated fair value                                                                                                 | 131,817       |          |
| Residential mortgage-backed securities - Private [Member]                                                            |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 19,770        | 22,649        |
| Gross unrealized gains                                                                                               | 982           | 750           |
| Gross unrealized losses                                                                                              | 0             | 454           |
| Estimated fair value                                                                                                 | 20,752        | 22,945        |
| Commercial mortgage-backed securities - GSE [Member]                                                                 |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 22,767        | 23,150        |
| Gross unrealized gains                                                                                               | 0             | 209           |
| Gross unrealized losses                                                                                              | 1,328         | 0             |
| Estimated fair value                                                                                                 | 21,439        | 23,359        |
| Commercial mortgage-backed securities - Private [Member]                                                             |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 10,408        | 5,283         |
| Gross unrealized gains                                                                                               | 0             | 34            |
| Gross unrealized losses                                                                                              | 823           | 0             |
| Estimated fair value                                                                                                 | 9,585         | 5,317         |
| **Held-to-maturity Securities [Abstract]**                                                                           |          |          |
| Amortized cost                                                                                                       | 10,071        |          |
| Gross unrealized gains                                                                                               | 0             |          |
| Gross unrealized losses                                                                                              | 763           |          |
| Estimated fair value                                                                                                 | 9,308         |          |
| Business Development Company investment [Member]                                                                     |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 1,753         |          |
| Gross unrealized gains                                                                                               | 984           |          |
| Gross unrealized losses                                                                                              | 0             |          |
| Estimated fair value                                                                                                 | 2,737         |          |
| Corporate notes [Member]                                                                                             |          |          |
| **Available-for-sale Securities [Abstract]**                                                                         |          |          |
| Amortized cost                                                                                                       | 16,795        | 36,710        |
| Gross unrealized gains                                                                                               | 82            | 270           |
| Gross unrealized losses                                                                                              | 0             | 17            |
| Estimated fair value                                                                                                 | $ 16,877      | $ 36,963      |

| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale and Held-to-maturity Securities, Amortized Cost Basis
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale and Held-to-maturity Securities, Gross Unrealized Gains
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale and Held-to-maturity Securities, Gross Unrealized Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax
+ References
No definition available.
+ Details**Name:**cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Held-to-maturity Securities, Unrecognized Holding Gain
+ References
No definition available.
+ Details**Name:**cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of debt and equity financial instruments classified as either held-to-maturity or available-for-sale.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.
+ References
No definition available.
+ Details**Name:**us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of investments in debt securities classified as held-to-maturity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563
+ Details**Name:**us-gaap_HeldToMaturitySecuritiesFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                |
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