| **Loans and Allowance for Loan Losses (Loans Sold) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|----------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable [Line Items]**                                                                          |           |           |   |   |
| Number of loans sold                                                                                           | 0              | 1              | 0 | 9 |
| Recorded investment of loans sold                                                                              | $ 0            | $ 602          | $ 0 | $ 6,267 |
| Contract pricing of loans sold                                                                                 | 0              | 540            | 0 | 6,486 |
| Commercial and agricultural [Member]                                                                           |           |           |   |   |
| **Financing Receivable [Line Items]**                                                                          |           |           |   |   |
| Recorded investment of loans sold                                                                              |           |           |   | 0 |
| Contract pricing of loans sold                                                                                 |           |           |   | 0 |
| Real estate - construction [Member]                                                                            |           |           |   |   |
| **Financing Receivable [Line Items]**                                                                          |           |           |   |   |
| Recorded investment of loans sold                                                                              |           |           |   | 0 |
| Contract pricing of loans sold                                                                                 |           |           |   | 0 |
| Real estate - mortgage, 1-4 family residential [Member]                                                        |           |           |   |   |
| **Financing Receivable [Line Items]**                                                                          |           |           |   |   |
| Number of loans sold                                                                                           |           | 1              |   | 7 |
| Recorded investment of loans sold                                                                              |           | 602            |   | 1,865 |
| Contract pricing of loans sold                                                                                 |           | 540            |   | 1,896 |
| Real estate - mortgage, commercial [Member]                                                                    |           |           |   |   |
| **Financing Receivable [Line Items]**                                                                          |           |           |   |   |
| Number of loans sold                                                                                           |           |           |   | 2 |
| Recorded investment of loans sold                                                                              |           | 0              |   | 4,402 |
| Contract pricing of loans sold                                                                                 |           | 0              |   | 4,590 |
| Consumer [Member]                                                                                              |           |           |   |   |
| **Financing Receivable [Line Items]**                                                                          |           |           |   |   |
| Recorded investment of loans sold                                                                              |           |           |   | 0 |
| Contract pricing of loans sold                                                                                 |           |           |   | $ 0 |

| X                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of loans sold during the period.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesReceivablesNumberOfLoansSold**Namespace Prefix:**cob_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate sale (contract price) amount of loans receivable that were sold to other entities. Includes mortgage and all other loans (collectively, loans) not classified as held for investment. Such loans are stated at the lower of cost or market, or elected fair value. Mortgage loans exclude mortgage-backed securities which are considered a debt security (other loan).
+ References
No definition available.
+ Details**Name:**cob_LoansReceivableHeldForSaleSalesContractPrice**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate carrying amount of loans receivable that were sold to other entities. Includes mortgage and all other loans (collectively, loans) not classified as held for investment. Such loans are stated at the lower of cost or market, or elected fair value. Mortgage loans exclude mortgage-backed securities which are considered a debt security (other loan).
+ References
No definition available.
+ Details**Name:**cob_LoansReceivableHeldForSaleSalesRecordedInvestmentSales**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |