| **Loans and Allowance for Loan Losses (Troubled Debt Restructuring) (Details) $ in Thousands** | 3 Months Ended | 9 Months Ended |
|------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                           |           |           |   |   |
| Number of loans | loans                                                                        | 5              | 2              | 7 | 19 |
| Pre-modification outstanding recorded investment                                               | $ 968          | $ 297          | $ 1,783 | $ 4,681 |
| Post-modification outstanding recorded investment                                              | $ 968          | $ 297          | $ 1,783 | $ 4,757 |
| Loans modified, extended terms | loans                                                         |           |           |   | 2 |
| Loans modified, interest rate modification | loans                                             |           |           | 1 | 17 |
| Commercial and agricultural [Member]                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                           |           |           |   |   |
| Number of loans | loans                                                                        | 0              | 0              | 0 | 2 |
| Pre-modification outstanding recorded investment                                               | $ 0            | $ 0            | $ 0 | $ 94 |
| Post-modification outstanding recorded investment                                              | $ 0            | $ 0            | $ 0 | $ 94 |
| Real estate - construction [Member]                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                           |           |           |   |   |
| Number of loans | loans                                                                        | 2              | 1              | 3 | 2 |
| Pre-modification outstanding recorded investment                                               | $ 280          | $ 37           | $ 649 | $ 523 |
| Post-modification outstanding recorded investment                                              | $ 280          | $ 37           | $ 649 | $ 523 |
| Real estate - mortgage, 1-4 family residential [Member]                                        |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                           |           |           |   |   |
| Number of loans | loans                                                                        | 2              | 1              | 3 | 8 |
| Pre-modification outstanding recorded investment                                               | $ 61           | $ 260          | $ 507 | $ 1,012 |
| Post-modification outstanding recorded investment                                              | $ 61           | $ 260          | $ 507 | $ 1,088 |
| Real estate - mortgage, commercial [Member]                                                    |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                           |           |           |   |   |
| Number of loans | loans                                                                        | 1              | 0              | 1 | 7 |
| Pre-modification outstanding recorded investment                                               | $ 627          | $ 0            | $ 627 | $ 3,052 |
| Post-modification outstanding recorded investment                                              | $ 627          | $ 0            | $ 627 | $ 3,052 |
| Consumer [Member]                                                                              |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                           |           |           |   |   |
| Number of loans | 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, 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                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_FinancingReceivableModificationsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cob_CommercialAndAgriculturalPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cob_RealEstateConstructionPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |