| **Loans and Allowance for Loan Losses (Troubled Debt Restructuring) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 1              | 11 | 20 |
| Pre-modification outstanding recorded investment                                                                                | $ 0            | $ 202          | $ 4,344 | $ 4,142 |
| Post-modification outstanding recorded investment                                                                               | 0              | 202            | 3,825 | 3,216 |
| Commercial and agricultural [Member]                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 0              | 0 |   |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0              | 0 |   |
| Post-modification outstanding recorded investment                                                                               | 0              | 0              | 0 |   |
| Real estate - construction [Member]                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 0              | 0 | 9 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0              | 0 | 2,526 |
| Post-modification outstanding recorded investment                                                                               | 0              | 0              | 0 | 1,603 |
| Real estate - mortgage, 1-4 family residential [Member]                                                                         |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 1              | 9 | 3 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 202            | 2,733 | 260 |
| Post-modification outstanding recorded investment                                                                               | 0              | 202            | 2,630 | 260 |
| Real estate - mortgage, commercial [Member]                                                                                     |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 0              | 2 | 8 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0              | 1,611 | 1,356 |
| Post-modification outstanding recorded investment                                                                               | 0              | 0              | 1,195 | 1,353 |
| Consumer [Member]                                                                                                               |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 0              | 0 |   |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0              | 0 |   |
| Post-modification outstanding recorded investment                                                                               | $ 0            | $ 0            | $ 0 |   |

| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings.
+ References
No definition available.
+ Details**Name:**cob_FinancingReceivableModificationsNumberOfContractsAlt**Namespace Prefix:**cob_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in financing receivables (after modification) modified by troubled debt restructurings.
+ References
No definition available.
+ Details**Name:**cob_FinancingReceivableModificationsPostModificationRecordedInvestmentAlt**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in financing receivables (before modification) modified by troubled debt restructurings.
+ References
No definition available.
+ Details**Name:**cob_FinancingReceivableModificationsPreModificationRecordedInvestmentAlt**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableModificationsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |