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Derivatives and Financial Instruments (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Derivative [Line Items]    
Number of interest rate swaps   2us-gaap_NumberOfInterestRateDerivativesHeld
Not designated as hedges [Member]    
Derivative [Line Items]    
Gain (loss) recognized in income $ 72,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 $ 3,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Not designated as hedges [Member] | Mortgage loan rate lock commitments [Member]    
Derivative [Line Items]    
Gain (loss) recognized in income (7,000)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateLockCommitmentsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 2,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateLockCommitmentsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Not designated as hedges [Member] | Mortgage loan forward sales and MBS [Member]    
Derivative [Line Items]    
Gain (loss) recognized in income 79,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 1,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForwardContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Federal Home Loan Bank Advances [Member] | Interest Rate Swap [Member]    
Derivative [Line Items]    
Notional amount   40,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
Federal Home Loan Bank Advances [Member] | Interest Rate Swap 1 [Member]    
Derivative [Line Items]    
Notional amount   20,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= cob_InterestRateSwap1Member
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
Federal Home Loan Bank Advances [Member] | Interest Rate Swap 2 [Member]    
Derivative [Line Items]    
Notional amount   20,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= cob_InterestRateSwap2Member
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
Federal Home Loan Bank Advances [Member] | Designated as hedging instrument [Member] | Interest Rate Swap [Member]    
Derivative [Line Items]    
Gain (loss) recognized in income $ (321,000)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_FederalHomeLoanBankAdvancesMember		 $ 0us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_FederalHomeLoanBankAdvancesMember