| **Loans (Details) (Real estate - mortgage, 1-4 family residential [Member], PC [Member], Granite Corp [Member], USD $)In Millions, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Real estate - mortgage, 1-4 family residential [Member] | PC [Member] | Granite Corp [Member]                                                                   |           |           |
| **Financing Receivable [Line Items]**                                                                                                                           |           |           |
| Loans purchases                                                                                                                                                 | $ 119.0        | $ 137.4        |
| Loans purchases premium                                                                                                                                         | $ 1.6          | $ 3.7          |

| X                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchased principal amount of loans and leases receivable during the period.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesReceivablePurchases**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Premium amount related to the purchased loans and leases receivable during the period.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesReceivablePurchasesPremium**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 |