| **Loans and Allowance for Loan Losses (Troubled Debt Restructuring) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |   |
| Number of loans                                                                                                                 | 6              | 5 |
| Pre-modification outstanding recorded investment                                                                                | $ 746          | $ 2,928 |
| Post-modification outstanding recorded investment                                                                               | 743            | 2,580 |
| Loans modified, extended terms                                                                                                  | 2              |   |
| Loans modified, interest rate modification                                                                                      | 4              |   |
| Commercial and agricultural [Member]                                                                                            |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |   |
| Number of loans                                                                                                                 | 1              | 0 |
| Pre-modification outstanding recorded investment                                                                                | 11             | 0 |
| Post-modification outstanding recorded investment                                                                               | 11             | 0 |
| Real estate - construction [Member]                                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |   |
| Number of loans                                                                                                                 | 0              | 0 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0 |
| Post-modification outstanding recorded investment                                                                               | 0              | 0 |
| Real estate - mortgage, 1-4 family residential [Member]                                                                         |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |   |
| Number of loans                                                                                                                 | 5              | 4 |
| Pre-modification outstanding recorded investment                                                                                | 735            | 1,777 |
| Post-modification outstanding recorded investment                                                                               | 732            | 1,773 |
| Real estate - mortgage, commercial [Member]                                                                                     |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |   |
| Number of loans                                                                                                                 | 0              | 1 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 1,151 |
| Post-modification outstanding recorded investment                                                                               | 0              | 807 |
| Consumer [Member]                                                                                                               |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |   |
| Number of loans                                                                                                                 | 0              | 0 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0 |
| Post-modification outstanding recorded investment                                                                               | $ 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
Financing Receivable, Modifications, Number of Contracts, Extended Terms
+ References
No definition available.
+ Details**Name:**cob_FinancingReceivableModificationsNumberofContractsExtendedTerms**Namespace Prefix:**cob_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Number of Contracts, Interest Rate
+ References
No definition available.
+ Details**Name:**cob_FinancingReceivableModificationsNumberofContractsInterestRate**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 |