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Off-Balance Sheet Arrangements (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Guarantees the performance of a customer to a third party [Member]    
Other Commitments [Line Items]    
Maximum potential amount of undiscounted future payments related to standby letters of credit $ 2.7us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_PerformanceGuaranteeMember
$ 0.7us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_PerformanceGuaranteeMember
Agreement to extend credit and undisbursed advances on customer lines of credit [Member]    
Other Commitments [Line Items]    
Commitments $ 286.3us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= cob_AgreementToExtendCreditAndUndisbursedAdvancesMember