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Investment Securities (Other Disclosures) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
securities
Dec. 31, 2014
securities
Schedule of Available-for-sale Securities [Line Items]    
FHLB stock $ 4.9us-gaap_FederalHomeLoanBankStock $ 4.9us-gaap_FederalHomeLoanBankStock
FRBR stock 9.6us-gaap_FederalReserveBankStock 4.7us-gaap_FederalReserveBankStock
Investment securities portfolio available as lendable collateral 200.8cob_AvailableForSaleSecuritiesAvailableCollateral  
Available-for-sale securities in an unrealized loss position 15us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 33us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Deposits [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Investment securities portfolio pledged as collateral 120.2us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
/ cob_CollateralAxis
= us-gaap_DepositsMember
 
Retail repurchase agreements [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Investment securities portfolio pledged as collateral 16.5us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
/ cob_CollateralAxis
= us-gaap_RepurchaseAgreementsMember
 
Others [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Investment securities portfolio pledged as collateral $ 170.9us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
/ cob_CollateralAxis
= cob_OthersMember