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Investment Securities (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Available-for-sale Securities [Abstract]    
Amortized cost $ 370,038us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 354,924us-gaap_AvailableForSaleSecuritiesAmortizedCost [1]
Gross unrealized gains 1,706cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax 1,282cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
Gross unrealized losses 3,902cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax 6,166cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
Estimated fair value 367,842us-gaap_AvailableForSaleSecurities 350,040us-gaap_AvailableForSaleSecurities [1]
Held-to-maturity Securities [Abstract]    
Amortized cost 140,559us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment 142,461us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Gross unrealized gains 772cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain 116cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Gross unrealized losses 372cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 1,702cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Estimated fair value 140,959us-gaap_HeldToMaturitySecuritiesFairValue 140,875us-gaap_HeldToMaturitySecuritiesFairValue [1]
Amortized cost, Total 510,597cob_AvailableForSaleAndHeldToMaturitySecuritiesAmortizedCostBasis 497,385cob_AvailableForSaleAndHeldToMaturitySecuritiesAmortizedCostBasis
Gross unrealized gains, Total 2,478cob_AvailableForSaleAndHeldToMaturitySecuritiesGrossUnrealizedGains 1,398cob_AvailableForSaleAndHeldToMaturitySecuritiesGrossUnrealizedGains
Gross unrealized losses, Total 4,274cob_AvailableForSaleAndHeldToMaturitySecuritiesGrossUnrealizedLosses 7,868cob_AvailableForSaleAndHeldToMaturitySecuritiesGrossUnrealizedLosses
Estimated fair value, Total 508,801us-gaap_AvailableForSaleSecuritiesAndHeldToMaturitySecurities 490,915us-gaap_AvailableForSaleSecuritiesAndHeldToMaturitySecurities
Obligations of U.S. government sponsored enterprises [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 2,022us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 2,028us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Gross unrealized gains 14cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 16cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Gross unrealized losses 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Estimated fair value 2,036us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 2,044us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Residential mortgage-backed securities - GSE [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 288,622us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 295,300us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Gross unrealized gains 899cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 438cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Gross unrealized losses 3,802cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 5,593cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Estimated fair value 285,719us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 290,145us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Held-to-maturity Securities [Abstract]    
Amortized cost 130,496us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 132,396us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Gross unrealized gains 590cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 116cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Gross unrealized losses 372cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 1,635cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Estimated fair value 130,714us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 130,877us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Residential mortgage-backed securities - Private [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 15,744us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 16,455us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross unrealized gains 690cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 820cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross unrealized losses 4cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 4cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Estimated fair value 16,430us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 17,271us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Commercial mortgage-backed securities - GSE [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 22,279us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		 22,377us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember
Gross unrealized gains 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember
Gross unrealized losses 71cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		 419cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember
Estimated fair value 22,208us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		 21,958us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember
Commercial mortgage-backed securities - Private [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 10,354us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 10,365us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross unrealized gains 103cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross unrealized losses 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 150cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Estimated fair value 10,457us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 10,215us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Held-to-maturity Securities [Abstract]    
Amortized cost 10,063us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 10,065us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross unrealized gains 182cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 0cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross unrealized losses 0cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 67cob_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Estimated fair value 10,245us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 9,998us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Corporate notes [Member]    
Available-for-sale Securities [Abstract]    
Amortized cost 31,017us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		 8,399us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember
Gross unrealized gains 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		 8cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember
Gross unrealized losses 25cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		 0cob_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember
Estimated fair value $ 30,992us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		 $ 8,407us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember
[1] Derived from audited consolidated financial statements