| **Loans and Allowance for Loan Losses (Allowance for Loan Losses, by Segment Portfolio) (Details) - USD ($) $ in Thousands** | 3 Months Ended | 9 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Allowance for Credit Losses [Roll Forward]**                                                         |           |           |   |   |
| Balance, beginning of period                                                                                                 | $ 17,989       | $ 23,975       | $ 20,345 | $ 26,785 |
| Charge-offs                                                                                                                  | (1,293)        | (1,970)        | (3,855) | (5,980) |
| Recoveries                                                                                                                   | 556            | 1,199          | 2,688 | 4,768 |
| Provision (recovery of provision)                                                                                            | (64)           | (1,679)        | (1,990) | (4,048) |
| Balance, end of period                                                                                                       | 17,188         | 21,525         | 17,188 | 21,525 |
| Commercial and agricultural [Member]                                                                                         |           |           |   |   |
| **Financing Receivable, Allowance for Credit Losses [Roll Forward]**                                                         |           |           |   |   |
| Balance, beginning of period                                                                                                 | 2,712          | 3,508          | 3,915 | 2,931 |
| Charge-offs                                                                                                                  | 0              | (301)          | (306) | (1,311) |
| Recoveries                                                                                                                   | 203            | 171            | 852 | 825 |
| Provision (recovery of provision)                                                                                            | (251)          | (58)           | (1,797) | 875 |
| Balance, end of period                                                                                                       | 2,664          | 3,320          | 2,664 | 3,320 |
| Real estate - construction [Member]                                                                                          |           |           |   |   |
| **Financing Receivable, Allowance for Credit Losses [Roll Forward]**                                                         |           |           |   |   |
| Balance, beginning of period                                                                                                 | 2,207          | 4,778          | 3,163 | 5,233 |
| Charge-offs                                                                                                                  | (51)           | (52)           | (136) | (676) |
| Recoveries                                                                                                                   | 120            | 131            | 495 | 1,458 |
| Provision (recovery of provision)                                                                                            | (245)          | (1,089)        | (1,491) | (2,247) |
| Balance, end of period                                                                                                       | 2,031          | 3,768          | 2,031 | 3,768 |
| Real estate - mortgage, 1-4 family residential [Member]                                                                      |           |           |   |   |
| **Financing Receivable, Allowance for Credit Losses [Roll Forward]**                                                         |           |           |   |   |
| Balance, beginning of period                                                                                                 | 5,517          | 7,410          | 5,847 | 8,869 |
| Charge-offs                                                                                                                  | (393)          | (234)          | (769) | (869) |
| Recoveries                                                                                                                   | 103            | 396            | 412 | 894 |
| Provision (recovery of provision)                                                                                            | (46)           | (1,217)        | (309) | (2,539) |
| Balance, end of period                                                                                                       | 5,181          | 6,355          | 5,181 | 6,355 |
| Real estate - mortgage, commercial [Member]                                                                                  |           |           |   |   |
| **Financing Receivable, Allowance for Credit Losses [Roll Forward]**                                                         |           |           |   |   |
| Balance, beginning of period                                                                                                 | 3,306          | 5,666          | 4,179 | 7,195 |
| Charge-offs                                                                                                                  | (33)           | (511)          | (301) | (1,307) |
| Recoveries                                                                                                                   | 56             | 123            | 442 | 716 |
| Provision (recovery of provision)                                                                                            | (194)          | (150)          | (1,185) | (1,476) |
| Balance, end of period                                                                                                       | 3,135          | 5,128          | 3,135 | 5,128 |
| Consumer [Member]                                                                                                            |           |           |   |   |
| **Financing Receivable, Allowance for Credit Losses [Roll Forward]**                                                         |           |           |   |   |
| Balance, beginning of period                                                                                                 | 4,247          | 2,613          | 3,241 | 2,557 |
| Charge-offs                                                                                                                  | (816)          | (872)          | (2,343) | (1,817) |
| Recoveries                                                                                                                   | 74             | 378            | 487 | 875 |
| Provision (recovery of provision)                                                                                            | 672            | 835            | 2,792 | 1,339 |
| Balance, end of period                                                                                                       | $ 4,177        | $ 2,954        | $ 4,177 | $ 2,954 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A valuation allowance for financing receivables that are expected to be uncollectible.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLosses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of recovery of financing receivables doubtful of collection that were previously charged off.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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:**us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.11) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585
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| X                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cob_CommercialAndAgriculturalPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cob_RealEstateConstructionPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |