| **Derivatives and Financial Instruments (Details)**                                                           | 3 Months Ended | 6 Months Ended |             |            |
|---------------------------------------------------------------------------------------------------------------|----------------|----------------|-------------|------------|
||
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Number of interest rate swaps | swap                                                                          |           |           |        |       | 2 |   |
| Derivatives not designated as hedging instruments [Member]                                                    |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Gain (Loss) Recognized                                                                                        | $ 64,000       | $ (57,000)     | $ 72,000    | $ 14,000   |   |   |
| Mortgage loan rate lock commitments [Member] | Derivatives not designated as hedging instruments [Member]     |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Gain (Loss) Recognized                                                                                        | 3,000          | 4,000          | 4,000       | (3,000)    |   |   |
| Mortgage loan forward sales [Member] | Derivatives not designated as hedging instruments [Member]             |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Gain (Loss) Recognized                                                                                        | 61,000         | (61,000)       | 68,000      | 17,000     |   |   |
| FHLB advances [Member] | Interest Rate Swap [Member]                                                          |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Notional amount                                                                                               |           |           |        |       | $ 40,000,000 |   |
| FHLB advances [Member] | Interest Rate Swap [Member] | Derivatives designated as hedging instruments [Member] |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Gain (Loss) Recognized                                                                                        | $ (137,000)    | $ 262,000      | $ (686,000) | $ (59,000) |   |   |
| FHLB advances [Member] | Interest Rate Swap 1 [Member]                                                        |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Notional amount                                                                                               |           |           |        |       | $ 20,000,000 |   |
| FHLB advances [Member] | Interest Rate Swap 2 [Member]                                                        |           |           |        |       |   |   |
| **Derivative [Line Items]**                                                                                   |           |           |        |       |   |   |
| Notional amount                                                                                               |           |           |        |       |   | $ 20,000,000 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B
+ Details**Name:**invest_DerivativeNotionalAmount**Namespace Prefix:**invest_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959
+ Details**Name:**us-gaap_DerivativeGainLossOnDerivativeNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_DerivativeLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of interest rate derivative instruments held by the entity at the reporting date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5579245-113959
+ Details**Name:**us-gaap_NumberOfInterestRateDerivativesHeld**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_UnderlyingAssetClassAxis=us-gaap_FederalHomeLoanBankAdvancesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=cob_InterestRateSwap1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=cob_InterestRateSwap2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |