| **Loans and Allowance for Loan Losses (Troubled Debt Restructuring) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 1              | 6              | 7 | 11 |
| Pre-modification outstanding recorded investment                                                                                | $ 17           | $ 1,489        | $ 752 | $ 4,417 |
| Post-modification outstanding recorded investment                                                                               | 21             | 1,429          | 828 | 4,009 |
| Loans modified, extended terms                                                                                                  |           |           | 2 | 2 |
| Loans modified, interest rate modification                                                                                      |           |           | 5 | 1 |
| Loans modified, extended terms and interest rate modification                                                                   |           |           |   | 8 |
| Commercial and agricultural [Member]                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 0              | 0 | 0 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0              | 0 | 0 |
| Post-modification outstanding recorded investment                                                                               | 0              | 0              | 0 | 0 |
| Real estate - construction [Member]                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 1              | 0 | 1 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 114            | 0 | 114 |
| Post-modification outstanding recorded investment                                                                               | 0              | 114            | 0 | 114 |
| Real estate - mortgage, 1-4 family residential [Member]                                                                         |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 1              | 4              | 7 | 8 |
| Pre-modification outstanding recorded investment                                                                                | 17             | 915            | 752 | 2,692 |
| Post-modification outstanding recorded investment                                                                               | 21             | 894            | 828 | 2,667 |
| Real estate - mortgage, commercial [Member]                                                                                     |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 1              | 0 | 2 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 460            | 0 | 1,611 |
| Post-modification outstanding recorded investment                                                                               | 0              | 421            | 0 | 1,228 |
| Consumer [Member]                                                                                                               |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                            |           |           |   |   |
| Number of loans                                                                                                                 | 0              | 0              | 0 | 0 |
| Pre-modification outstanding recorded investment                                                                                | 0              | 0              | 0 | 0 |
| Post-modification outstanding recorded investment                                                                               | $ 0            | $ 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
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, Extended Terms and Interest Rate
+ References
No definition available.
+ Details**Name:**cob_FinancingReceivableModificationsNumberofContractsExtendedTermsandInterestRate**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                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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