| **Loans and Allowance for Loan Losses (Details) (USD $)**                                               | 3 Months Ended | 9 Months Ended |             |
|---------------------------------------------------------------------------------------------------------|----------------|----------------|-------------|
||
| **Financing Receivable [Line Items]**                                                                   |           |                |        |  |   |  |   |  |   |
| Allowance for Loan and Lease Losses, Write-offs                                                         | $ 1,724,000    |                | $ 9,276,000 |  | $ 10,836,000 |  | $ 23,580,000 |  |   |
| Allowance for Loan Losses, Recovery                                                                     | 2,376,000      |                | 1,552,000   |  | 8,206,000 |  | 4,202,000 |  |   |
| Allowance for Loan and Lease Losses, Net Write-downs                                                    | (652,000)      |                | 7,724,000   |  | 2,630,000 |  | 19,378,000 |  |   |
| Loans and Leases Receivable, Ratio, Allowance for Loan and Lease Losses to Loans and Leases Receivables | 2.12%          | [1]            | 2.51%       | [1] | 2.12% | [1] | 2.51% | [1] | 2.49% |
| Real estate - mortgage, 1-4 family residential [Member] | PC [Member] | Granite Corp [Member]           |           |                |        |  |   |  |   |  |   |
| **Financing Receivable [Line Items]**                                                                   |           |                |        |  |   |  |   |  |   |
| Loans purchases                                                                                         |           |                |        |  | 134,800,000 |  | 173,200,000 |  |   |
| Loans purchases premium                                                                                 |           |                |        |  | $ 1,000,000 |  | $ 4,000,000 |  |   |
|                                                                                                         |
| [1]Excludes discontinued operations.                                                                    |

| 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
Reflects the amount of loans and leases that have been removed, or charged off, from both loan receivables and the reserve for credit losses, typically because they are considered to be not salvageable or have been purchased by unconsolidated collection entities.
+ References
Reference 1: 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 2: 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=7512638&loc=d3e5212-111524Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 114 -Paragraph 20 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |