| **Loans and Allowance for Loan Losses (Granite Purchased Loans) (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2013 | Dec. 31, 2012 |
|-----------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Allowance of loan losses                                                                                                    | $ 26,039      | $ 26,785      | [1]           | $ 29,641      | $ 29,314 |
| Loans held for investment                                                                                                   | 1,219,785     | 1,212,248     | [1]           |          |   |
| PI [Member]                                                                                                                 |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Loans held for investment                                                                                                   | 149,033       | 161,652       |               |          |   |
| PC [Member]                                                                                                                 |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Loans held for investment                                                                                                   | 1,070,752     | 1,050,596     |               |          |   |
| Granite Corp [Member]                                                                                                       |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Purchased impaired                                                                                                          | 149,033       | 161,651       |               | 205,884       | 228,392 |
| Purchased contractual                                                                                                       | 29,450        | 28,448        |               |          |   |
| Loans held for investment                                                                                                   | 178,483       | 190,099       |               |          |   |
| Unpaid principal balance                                                                                                    | 188,451       | 202,174       |               |          |   |
| Granite Corp [Member] | Commercial and agricultural [Member]                                                                |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Purchased impaired                                                                                                          | 9,722         | 10,729        |               |          |   |
| Purchased contractual                                                                                                       | 4,044         | 5,948         |               |          |   |
| Loans held for investment                                                                                                   | 13,766        | 16,677        |               |          |   |
| Unpaid principal balance                                                                                                    | 12,654        | 16,452        |               |          |   |
| Granite Corp [Member] | Real estate - construction [Member]                                                                 |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Purchased impaired                                                                                                          | 11,200        | 9,892         |               |          |   |
| Purchased contractual                                                                                                       | 0             | 0             |               |          |   |
| Loans held for investment                                                                                                   | 11,200        | 9,892         |               |          |   |
| Unpaid principal balance                                                                                                    | 12,218        | 11,368        |               |          |   |
| Granite Corp [Member] | Real estate - mortgage, 1-4 family residential [Member]                                             |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Purchased impaired                                                                                                          | 20,399        | 26,854        |               |          |   |
| Purchased contractual                                                                                                       | 25,041        | 22,127        |               |          |   |
| Loans held for investment                                                                                                   | 45,440        | 48,981        |               |          |   |
| Unpaid principal balance                                                                                                    | 47,829        | 51,359        |               |          |   |
| Granite Corp [Member] | Real estate - mortgage, commercial [Member]                                                         |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Purchased impaired                                                                                                          | 106,552       | 113,178       |               |          |   |
| Purchased contractual                                                                                                       | 365           | 373           |               |          |   |
| Loans held for investment                                                                                                   | 106,917       | 113,551       |               |          |   |
| Unpaid principal balance                                                                                                    | 114,984       | 122,197       |               |          |   |
| Granite Corp [Member] | Consumer [Member]                                                                                   |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Purchased impaired                                                                                                          | 1,160         | 998           |               |          |   |
| Purchased contractual                                                                                                       | 0             | 0             |               |          |   |
| Loans held for investment                                                                                                   | 1,160         | 998           |               |          |   |
| Unpaid principal balance                                                                                                    | 766           | 798           |               |          |   |
| Granite Corp [Member] | PI [Member]                                                                                         |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Allowance of loan losses                                                                                                    | 5,200         | 5,600         |               |          |   |
| Granite Corp [Member] | PC [Member]                                                                                         |          |          |               |          |   |
| **Financing Receivable [Line Items]**                                                                                       |          |          |               |          |   |
| Allowance of loan losses                                                                                                    | $ 300         | $ 300         |               |          |   |
|                                                                                                                             |
| [1]Derived from audited consolidated financial statements                                                                   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The recorded balance before provision for allowance.  This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans and leases held in portfolio acquired with no evidence of credit deterioration, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of loans and leases receivables with related allowance for credit losses and without related allowance for credit losses.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesRecievableUnpaidPrincipalBalance**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section L -Subsection 1
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522
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