| **Investment Securities (Continuous Unrealized Loss Position) (Details) (USD $)In Thousands, unless otherwise specified** | Jun. 30, 2014securities | Dec. 31, 2013securities |
|---------------------------------------------------------------------------------------------------------------------------|-------------------------|-------------------------|
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | $ 14                    | $ 232,060               |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 17,408                  |
| 12 months or more, estimated fair value                                                                                   | 332,532                 | 97,940                  |
| 12 months or more, gross unrealized losses                                                                                | 13,061                  | 9,083                   |
| Total, estimated fair value                                                                                               | 332,546                 | 330,000                 |
| Total, gross unrealized losses                                                                                            | 13,061                  | 26,491                  |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 0                       | 141,125                 |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 10,670                  |
| 12 months or more, estimated fair value                                                                                   | 142,288                 | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 4,767                   | 0                       |
| Total, estimated fair value                                                                                               | 142,288                 | 141,125                 |
| Total, gross unrealized losses                                                                                            | 4,767                   | 10,670                  |
| **Continuous Unrealized Loss Position [Abstract]**                                                                        |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 14                      | 373,185                 |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 28,078                  |
| 12 months or more, estimated fair value                                                                                   | 474,820                 | 97,940                  |
| 12 months or more, gross unrealized losses                                                                                | 17,828                  | 9,083                   |
| Total, estimated fair value                                                                                               | 474,834                 | 471,125                 |
| Total, gross unrealized losses                                                                                            | 17,828                  | 37,161                  |
| 12 months or more, number of securities                                                                                   | 49                      | 10                      |
| Residential mortgage-backed securities - GSE [Member]                                                                     |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 14                      | 222,475                 |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 16,585                  |
| 12 months or more, estimated fair value                                                                                   | 300,334                 | 76,501                  |
| 12 months or more, gross unrealized losses                                                                                | 12,298                  | 7,755                   |
| Total, estimated fair value                                                                                               | 300,348                 | 298,976                 |
| Total, gross unrealized losses                                                                                            | 12,298                  | 24,340                  |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 0                       | 131,817                 |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 9,907                   |
| 12 months or more, estimated fair value                                                                                   | 132,398                 | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 4,589                   | 0                       |
| Total, estimated fair value                                                                                               | 132,398                 | 131,817                 |
| Total, gross unrealized losses                                                                                            | 4,589                   | 9,907                   |
| 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                                                                                   | 22,052                  | 21,439                  |
| 12 months or more, gross unrealized losses                                                                                | 522                     | 1,328                   |
| Total, estimated fair value                                                                                               | 22,052                  | 21,439                  |
| Total, gross unrealized losses                                                                                            | 522                     | 1,328                   |
| Commercial mortgage-backed securities - Private [Member]                                                                  |                    |                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                         |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 0                       | 9,585                   |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 823                     |
| 12 months or more, estimated fair value                                                                                   | 10,146                  | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 241                     | 0                       |
| Total, estimated fair value                                                                                               | 10,146                  | 9,585                   |
| Total, gross unrealized losses                                                                                            | 241                     | 823                     |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                           |                    |                    |
| Less than 12 months, estimated fair value                                                                                 | 0                       | 9,308                   |
| Less than 12 months, gross unrealized losses                                                                              | 0                       | 763                     |
| 12 months or more, estimated fair value                                                                                   | 9,890                   | 0                       |
| 12 months or more, gross unrealized losses                                                                                | 178                     | 0                       |
| Total, estimated fair value                                                                                               | 9,890                   | 9,308                   |
| Total, gross unrealized losses                                                                                            | $ 178                   | $ 763                   |

| X                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: 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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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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