| **Loans and Allowance for Loan Losses (Details) (USD $)**                                               | 12 Months Ended                                                                                                                                                                                                                                                      |
|---------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Financing Receivable [Line Items]**                                                                   |                                                                                                                                                                                                                                                                 |   |   |
| Allowance for Loan and Lease Losses, Write-offs                                                         | $ 7,703,000us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs                                                                                                                                                                                                           | $ 13,344,000us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs | $ 30,968,000us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs |
| Allowance for Loan Losses, Recovery                                                                     | 6,634,000cob_AllowanceForLoanLossesRecovery                                                                                                                                                                                                                          | 10,292,000cob_AllowanceForLoanLossesRecovery | 6,873,000cob_AllowanceForLoanLossesRecovery |
| Allowance for Loan and Lease Losses, Net Write-downs                                                    | 1,069,000cob_AllowanceForLoanAndLeaseLossesNetWriteDowns                                                                                                                                                                                                             | 3,052,000cob_AllowanceForLoanAndLeaseLossesNetWriteDowns | 24,095,000cob_AllowanceForLoanAndLeaseLossesNetWriteDowns |
| Loans and Leases Receivable, Ratio, Allowance for Loan and Lease Losses to Loans and Leases Receivables | 1.50%cob_LoansAndLeasesReceivableRatioAllowanceForLoanAndLeaseLossesToLoansAndLeasesReceivables                                                                                                                                                                      | 2.21%cob_LoansAndLeasesReceivableRatioAllowanceForLoanAndLeaseLossesToLoansAndLeasesReceivables | 2.41%cob_LoansAndLeasesReceivableRatioAllowanceForLoanAndLeaseLossesToLoansAndLeasesReceivables |
| Real estate - mortgage, 1-4 family residential [Member] | PC [Member] | Granite Corp. [Member]          |                                                                                                                                                                                                                                                                 |   |   |
| **Financing Receivable [Line Items]**                                                                   |                                                                                                                                                                                                                                                                 |   |   |
| Loans purchases                                                                                         | 19,700,000cob_LoansAndLeasesReceivablePurchases/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember/ us-gaap_InternalCreditAssessmentAxis= us-gaap_PerformingFinancingReceivableMember       | 157,600,000cob_LoansAndLeasesReceivablePurchases/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember/ us-gaap_InternalCreditAssessmentAxis= us-gaap_PerformingFinancingReceivableMember |   |
| Loans purchases premium                                                                                 | $ 300,000cob_LoansAndLeasesReceivablePurchasesPremium/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember/ us-gaap_InternalCreditAssessmentAxis= us-gaap_PerformingFinancingReceivableMember | $ 1,300,000cob_LoansAndLeasesReceivablePurchasesPremium/ us-gaap_BusinessAcquisitionAxis= cob_GraniteCorpMember/ us-gaap_FinancialInstrumentAxis= us-gaap_ResidentialMortgageMember/ us-gaap_InternalCreditAssessmentAxis= us-gaap_PerformingFinancingReceivableMember |   |

| 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 |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |