| **Other Real Estate Owned and Property Acquired in Settlement of Loans (Real Estate Owned) (Details) (USD $)** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Real Estate Acquired Through Foreclosure [Roll Forward]**                                                    |           |           |   |   |
| Real estate acquired in settlement of loans, beginning of period                                               | $ 24,573,000   | $ 46,307,000   | $ 28,353,000 | $ 62,796,000 |
| Plus: New real estate acquired in settlement of loans                                                          | 983,000        | 2,824,000      | 1,251,000 | 6,793,000 |
| Less: Sales of real estate acquired in settlement of loans                                                     | (3,154,000)    | (10,422,000)   | (6,928,000) | (30,661,000) |
| Less: Write-downs and net loss on sales charged to expense                                                     | (660,000)      | (3,047,000)    | (934,000) | (3,266,000) |
| Real estate acquired in settlement of loans, end of period                                                     | 21,742,000     | 35,662,000     | 21,742,000 | 35,662,000 |
| Number of assets under contract for sale                                                                       | 18             |           | 18 |   |
| Net carrying value of assets under contract for sale                                                           | $ 4,300,000    |           | $ 4,300,000 |   |

| 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 FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5
+ Details**Name:**us-gaap_RealEstateAcquiredThroughForeclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |