| **Fair Values of Assets and Liabilities (Quantitative Information about Level 3 Fair Value Measurements) (Details) - Level 3 [Member] - USD ($) $ in Thousands** | 9 Months Ended | 12 Months Ended |         |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|---------|
||
| Discounted Appraisal [Member]                                                                                                                                    |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Impaired loans, net                                                                                                                                              | $ 3,104        |            | $ 7,433 |
| Other real estate owned                                                                                                                                          | $ 12,991       |            | 15,579  |
| Discounted Appraisal [Member] | Impaired loans [Member] | Lower range [Member]                                                                                   |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Collateral discounts                                                                                                                                             | 1.00%          | 1.00%           |    |
| Discounted Appraisal [Member] | Impaired loans [Member] | Upper range [Member]                                                                                   |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Collateral discounts                                                                                                                                             | 30.00%         | 30.00%          |    |
| Discounted Appraisal [Member] | Other real estate owned [Member] | Lower range [Member]                                                                          |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Collateral discounts                                                                                                                                             | 1.00%          | 1.00%           |    |
| Discounted Appraisal [Member] | Other real estate owned [Member] | Upper range [Member]                                                                          |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Collateral discounts                                                                                                                                             | 30.00%         | 30.00%          |    |
| Discounted Cash Flows [Member]                                                                                                                                   |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Mortgage servicing rights                                                                                                                                        | $ 2,108        |            | $ 1,726 |
| Discounted Cash Flows [Member] | Mortgage servicing rights [Member] | Lower range [Member]                                                                       |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Prepayment rate                                                                                                                                                  | 10.00%         | 10.00%          |    |
| Discount rate                                                                                                                                                    | 6.00%          | 6.00%           |    |
| Discounted Cash Flows [Member] | Mortgage servicing rights [Member] | Upper range [Member]                                                                       |           |            |    |
| **Fair Value Inputs, Quantitative Information [Line Items]**                                                                                                     |           |            |    |
| Prepayment rate                                                                                                                                                  | 25.00%         | 25.00%          |    |
| Discount rate                                                                                                                                                    | 10.00%         | 10.00%          |    |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.
+ References
No definition available.
+ Details**Name:**cob_ForeclosedAssetFairValueDisclosure**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
No definition available.
+ Details**Name:**cob_ImpairedFinancingReceivableFairValueDisclosure**Namespace Prefix:**cob_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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_FairValueInputsAssetsQuantitativeInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value.  For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsDiscountRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsPrepaymentRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=56950315&loc=d3e122625-111746
+ Details**Name:**us-gaap_ServicingAssetAtFairValueAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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