| **Other Real Estate Owned and Property Acquired in Settlement of Loans (Real Estate Owned) (Details) $ in Thousands** | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Real Estate Acquired Through Foreclosure [Roll Forward]**                                                           |           |           |   |   |
| Real estate acquired in settlement of loans, beginning of period                                                      | $ 19,717       | $ 21,742       | $ 20,122 | $ 28,353 |
| Plus: New real estate acquired in settlement of loans                                                                 | 766            | 1,643          | 4,199 | 2,894 |
| Less: Sales of real estate acquired in settlement of loans                                                            | (1,110)        | (3,400)        | (4,263) | (10,328) |
| Less: Write-downs and net loss on sales charged to expense                                                            | (494)          | 195            | (1,179) | (739) |
| Real estate acquired in settlement of loans, end of period                                                            | $ 18,879       | $ 20,180       | $ 18,879 | $ 20,180 |
| Number of assets under contract for sale | assets                                                                     | 18             |           | 18 |   |
| Net carrying value of assets under contract for sale                                                                  | $ 2,700        |           | $ 2,700 |   |
| Mortgage loans collateralized by residential real estate properties that are in the process of foreclosure            | 1,900          |           | 1,900 |   |
| Real estate - mortgage, 1-4 family residential [Member]                                                               |           |           |   |   |
| **Real Estate Acquired Through Foreclosure [Roll Forward]**                                                           |           |           |   |   |
| Real estate acquired in settlement of loans, end of period                                                            | $ 3,400        |           | $ 3,400 |   |

| 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                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
+ 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
Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 35 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL49126937-111524
+ Details**Name:**us-gaap_MortgageLoansInProcessOfForeclosureAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5162-111524Reference 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=28367877&loc=d3e4437-111522Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12Reference 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 5
+ Details**Name:**us-gaap_RealEstateAcquiredThroughForeclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |