| **Investment Securities (Continuous Unrealized Loss Position) (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2014securities | Dec. 31, 2013securities |
|---------------------------------------------------------------------------------------------------------------------------|-------------------------|-------------------------|
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | $ 175,453               | $ 232,060               |
| Less than 12 months, gross unrealized losses                                                                              | 9,538                   | 17,408                  |
| 12 months or more, estimated fair value                                                                                   | 155,290                 | 97,940                  |
| 12 months or more, gross unrealized losses                                                                                | 11,534                  | 9,083                   |
| Total, estimated fair value                                                                                               | 330,743                 | 330,000                 |
| Total, gross unrealized losses                                                                                            | 21,072                  | 26,491                  |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 141,002                 | 141,125                 |
| Less than 12 months, gross unrealized losses                                                                              | 8,058                   | 10,670                  |
| 12 months or more, estimated fair value                                                                                   | 0                       | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 0                       | 0                       |
| Total, estimated fair value                                                                                               | 141,002                 | 141,125                 |
| Total, gross unrealized losses                                                                                            | 8,058                   | 10,670                  |
| **Continuous Unrealized Loss Position [Abstract]**                                                                        |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 316,455                 | 373,185                 |
| Less than 12 months, gross unrealized losses                                                                              | 17,596                  | 28,078                  |
| 12 months or more, estimated fair value                                                                                   | 155,290                 | 97,940                  |
| 12 months or more, gross unrealized losses                                                                                | 11,534                  | 9,083                   |
| Total, estimated fair value                                                                                               | 471,745                 | 471,125                 |
| Total, gross unrealized losses                                                                                            | 29,130                  | 37,161                  |
| 12 months or more, number of securities                                                                                   | 14                      | 10                      |
| Residential mortgage-backed securities - GSE [Member]                                                                     |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 164,313                 | 222,475                 |
| Less than 12 months, gross unrealized losses                                                                              | 8,975                   | 16,585                  |
| 12 months or more, estimated fair value                                                                                   | 133,699                 | 76,501                  |
| 12 months or more, gross unrealized losses                                                                                | 10,454                  | 7,755                   |
| Total, estimated fair value                                                                                               | 298,012                 | 298,976                 |
| Total, gross unrealized losses                                                                                            | 19,429                  | 24,340                  |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 131,427                 | 131,817                 |
| Less than 12 months, gross unrealized losses                                                                              | 7,564                   | 9,907                   |
| 12 months or more, estimated fair value                                                                                   | 0                       | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 0                       | 0                       |
| Total, estimated fair value                                                                                               | 131,427                 | 131,817                 |
| Total, gross unrealized losses                                                                                            | 7,564                   | 9,907                   |
| Residential mortgage-backed securities - Private [Member]                                                                 |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 1,297                   |                    |
| Less than 12 months, gross unrealized losses                                                                              | 9                       |                    |
| 12 months or more, estimated fair value                                                                                   | 0                       |                    |
| 12 months or more, gross unrealized losses                                                                                | 0                       |                    |
| Total, estimated fair value                                                                                               | 1,297                   |                    |
| Total, gross unrealized losses                                                                                            | 9                       |                    |
| Commercial mortgage-backed securities - GSE [Member]                                                                      |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 0                       | 0                       |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 0                       |
| 12 months or more, estimated fair value                                                                                   | 21,591                  | 21,439                  |
| 12 months or more, gross unrealized losses                                                                                | 1,080                   | 1,328                   |
| Total, estimated fair value                                                                                               | 21,591                  | 21,439                  |
| Total, gross unrealized losses                                                                                            | 1,080                   | 1,328                   |
| Commercial mortgage-backed securities - Private [Member]                                                                  |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 |                    | 9,585                   |
| Less than 12 months, gross unrealized losses                                                                              |                    | 823                     |
| 12 months or more, estimated fair value                                                                                   |                    | 0                       |
| 12 months or more, gross unrealized losses                                                                                |                    | 0                       |
| Total, estimated fair value                                                                                               |                    | 9,585                   |
| Total, gross unrealized losses                                                                                            |                    | 823                     |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 9,575                   | 9,308                   |
| Less than 12 months, gross unrealized losses                                                                              | 494                     | 763                     |
| 12 months or more, estimated fair value                                                                                   | 0                       | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 0                       | 0                       |
| Total, estimated fair value                                                                                               | 9,575                   | 9,308                   |
| Total, gross unrealized losses                                                                                            | 494                     | 763                     |
| Corporate notes [Member]                                                                                                  |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 9,843                   |                    |
| Less than 12 months, gross unrealized losses                                                                              | 554                     |                    |
| 12 months or more, estimated fair value                                                                                   | 0                       |                    |
| 12 months or more, gross unrealized losses                                                                                | 0                       |                    |
| Total, estimated fair value                                                                                               | 9,843                   |                    |
| Total, gross unrealized losses                                                                                            | $ 554                   |                    |

| X                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
+ References
No definition available.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This represents the number 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
No definition available.
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| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses
+ References
No definition available.
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| X                                                                                                                                                                    |
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| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Aggregate Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Fair Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
+ References
No definition available.
+ Details**Name:**cob_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
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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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.
+ References
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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
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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
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