| **Loans and Allowance for Loan Losses (Details) (USD $)**                                               | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable [Line Items]**                                                                   |            |   |   |
| Allowance for Loan and Lease Losses, Write-offs                                                         | $ 13,344,000    | $ 30,968,000 | $ 128,424,000 |
| Allowance for Loan Losses, Recovery                                                                     | 10,292,000      | 6,873,000 | 6,735,000 |
| Allowance for Loan and Lease Losses, Net Write-downs                                                    | 3,052,000       | 24,095,000 | 121,689,000 |
| Loans and Leases Receivable, Ratio, Allowance for Loan and Lease Losses to Loans and Leases Receivables | 2.21%           | 2.49% | 3.23% |
| Real estate - mortgage, 1-4 family residential [Member] | PC [Member] | Granite Corp. [Member]          |            |   |   |
| **Financing Receivable [Line Items]**                                                                   |            |   |   |
| Loans purchases                                                                                         | 157,600,000     | 173,200,000 |   |
| Loans purchases premium                                                                                 | $ 1,300,000     | $ 4,000,000 |   |

| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net direct write-downs (net of recoveries) of financing receivables charged against the allowance.
+ References
No definition available.
+ Details**Name:**cob_AllowanceForLoanAndLeaseLossesNetWriteDowns**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of recovery of loans and leases receivable doubtful of collection that were previously charged off.
+ References
No definition available.
+ Details**Name:**cob_AllowanceForLoanLossesRecovery**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Allowance for loan and lease losses to loans and leases receivables ratio.
+ References
No definition available.
+ Details**Name:**cob_LoansAndLeasesReceivableRatioAllowanceForLoanAndLeaseLossesToLoansAndLeasesReceivables**Namespace Prefix:**cob_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9
+ Details**Name:**us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |