| **Other Real Estate Owned and Property Acquired in Settlement of Loans (Real Estate Owned) (Details) (USD $)** | 3 Months Ended | 9 Months Ended |              |               |
|----------------------------------------------------------------------------------------------------------------|----------------|----------------|--------------|---------------|
||
| **Real Estate Acquired Through Foreclosure [Roll Forward]**                                                    |           |           |         |          |   |   |
| Real estate acquired in settlement of loans, beginning of period                                               |           |           | $ 62,796,000 | $ 110,009,000 | $ 35,662,000 | $ 86,183,000 |
| Plus: New real estate acquired in settlement of loans                                                          | 3,610,000      | 9,746,000      | 10,403,000   | 24,766,000    |   |   |
| Less: Sales of real estate acquired in settlement of loans                                                     | (6,411,000)    | (13,826,000)   | (37,072,000) | (38,202,000)  |   |   |
| Less: Write-downs and net loss on sales charged to expense                                                     | 262,000        | (1,621,000)    | (3,004,000)  | (16,091,000)  |   |   |
| Real estate acquired in settlement of loans, end of period                                                     | 33,123,000     | 80,482,000     | 33,123,000   | 80,482,000    | 35,662,000 | 86,183,000 |
| Number of assets under contract for sale                                                                       | 13             |           | 13           |          |   |   |
| Net carrying value of assets under contract for sale                                                           | $ 7,200,000    |           | $ 7,200,000  |          |   |   |

| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This amount represents new real estate acquired in settlement of loans (net of write downs) during the period.
+ References
No definition available.
+ Details**Name:**cob_RealEstateAcquiredThroughForeclosureAdditions**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cob_RealEstateAcquiredThroughForeclosureRollForward**Namespace Prefix:**cob_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Real Estate Acquired Through Foreclosure, Sale Commitment, Carrying Amount
+ References
No definition available.
+ Details**Name:**cob_RealEstateAcquiredThroughForeclosureSaleCommitmentCarryingAmount**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Real Estate Acquired Through Foreclosure, Sale Commitment, Number of Contracts
+ References
No definition available.
+ Details**Name:**cob_RealEstateAcquiredThroughForeclosureSaleCommitmentNumberOfContracts**Namespace Prefix:**cob_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This amount represents write-downs on the existing real estate acquired through foreclosure and net gain (loss) on the sales of real estate acquired through foreclosure during the period.
+ References
No definition available.
+ Details**Name:**cob_RealEstateAcquiredThroughForeclosureWritedownsAndNetGainLossOnSales**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This amount represents sale of real estate acquired in settlement of loans during the period.
+ References
No definition available.
+ Details**Name:**cob_SaleOfRealEstateAcquiredThroughForeclosure**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph f -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.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6375948&loc=d3e4437-111522Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-DEP -Paragraph 2, 9, 10, 11 -IssueDate 2006-05-01 -Chapter 11 -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.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12
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