| **Repurchase Agreement Borrowings (Details) - USD ($) $ in Thousands**                    | Jun. 30, 2016 | Dec. 31, 2015 |
|-------------------------------------------------------------------------------------------|---------------|---------------|
| **Assets Sold under Agreements to Repurchase [Line Items]**                               |          |          |  |
| Repurchase agreements                                                                     | $ 12,761      | $ 7,219       | [1] |
| Available unpledged securities                                                            | 181,000       | 213,600       |  |
| Residential mortgage-backed securities - GSE [Member]                                     |          |          |  |
| **Assets Sold under Agreements to Repurchase [Line Items]**                               |          |          |  |
| Repurchase agreements                                                                     | 12,761        | 7,219         |  |
| Market value of collateral                                                                | 15,900        | 17,100        |  |
| Residential mortgage-backed securities - GSE [Member] | Overnight and Continuous [Member] |          |          |  |
| **Assets Sold under Agreements to Repurchase [Line Items]**                               |          |          |  |
| Repurchase agreements                                                                     | 12,761        | 7,219         |  |
| Residential mortgage-backed securities - GSE [Member] | 1-90 Days [Member]                |          |          |  |
| **Assets Sold under Agreements to Repurchase [Line Items]**                               |          |          |  |
| Repurchase agreements                                                                     | 0             | 0             |  |
| Residential mortgage-backed securities - GSE [Member] | Over 90 Days [Member]             |          |          |  |
| **Assets Sold under Agreements to Repurchase [Line Items]**                               |          |          |  |
| Repurchase agreements                                                                     | $ 0           | $ 0           |  |
|                                                                                           |
| [1]Derived from audited consolidated financial statements                                 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This represents the amount of available-for-sale securities that are not currently pledged to any debts, agreements or any other arrangements and available as lendable collateral.
+ References
No definition available.
+ Details**Name:**cob_AvailableForSaleSecuritiesAvailableCollateral**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_AssetsSoldUnderAgreementsToRepurchaseLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(m)(1)(a)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(1)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49176635&loc=d3e9760-107771Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Repurchase Agreement Accounted for as a Collateralized Borrowing -URI http://asc.fasb.org/extlink&oid=49203159
+ Details**Name:**us-gaap_SecuritiesSoldUnderAgreementsToRepurchase**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of securities pledged as collateral against securities sold under agreement to repurchase.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226013-175313Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Repurchase Agreement Accounted for as a Collateralized Borrowing -URI http://asc.fasb.org/extlink&oid=49203159
+ Details**Name:**us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis=cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityOvernightMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=cob_Maturity1to90DaysMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityOver90DaysMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |