| **Loans and Allowance for Loan Losses (Nonaccrual Status) (Details) (USD $)**    | 3 Months Ended | 9 Months Ended |               |
|----------------------------------------------------------------------------------|----------------|----------------|---------------|
||
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Interest lost on nonaccrual loans                                                | $ 700,000      | $ 1,500,000    | $ 2,300,000   | $ 2,900,000 |   |  |
| Loans on nonaccrual status                                                       | 51,100,000     |           | 51,100,000    |   | 79,200,000 |  |
| Loans held for investment on nonaccrual status                                   | 51,131,000     |           | 51,131,000    |   | 79,242,000 |  |
| Net loans held for investment                                                    | 1,169,755,000  |           | 1,169,755,000 |   | 1,147,721,000 | [1] |
| Nonperforming [Member]                                                           |           |           |          |   |   |  |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Loans held for investment on nonaccrual status                                   | 51,131,000     |           | 51,131,000    |   | 79,242,000 |  |
| 90 or more days past due and accruing                                            | 76,000         |           | 76,000        |   | 227,000 |  |
| Net loans held for investment                                                    | 51,207,000     |           | 51,207,000    |   | 79,469,000 |  |
| Nonperforming [Member] | Commercial and agricultural [Member]                    |           |           |          |   |   |  |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Loans held for investment on nonaccrual status                                   | 590,000        |           | 590,000       |   | 2,746,000 |  |
| Nonperforming [Member] | Real estate - construction [Member]                     |           |           |          |   |   |  |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Loans held for investment on nonaccrual status                                   | 10,190,000     |           | 10,190,000    |   | 14,297,000 |  |
| Nonperforming [Member] | Real estate - mortgage, 1-4 family residential [Member] |           |           |          |   |   |  |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Loans held for investment on nonaccrual status                                   | 12,822,000     |           | 12,822,000    |   | 18,372,000 |  |
| Nonperforming [Member] | Real estate - mortgage, commercial [Member]             |           |           |          |   |   |  |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Loans held for investment on nonaccrual status                                   | 27,425,000     |           | 27,425,000    |   | 43,621,000 |  |
| Nonperforming [Member] | Consumer [Member]                                       |           |           |          |   |   |  |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**             |           |           |          |   |   |  |
| Loans held for investment on nonaccrual status                                   | $ 104,000      |           | $ 104,000     |   | $ 206,000 |  |
|                                                                                  |
| [1]Derived from audited consolidated financial statements                        |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans (including held-for-sale receivable) were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the carrying amount of loans, including loans held-for-sale, deemed to be questionable as to collection on which no interest is continuing to be recognized.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesReceivableIncludingHeldForSaleReceivableNonperformingNonaccrualOfInterest**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are 90 days or more past due and still accruing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5111-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5111-111524
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| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. 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=6375948&loc=d3e4428-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: 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) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
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