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Loans and Allowance for Loan Losses (Unfunded Commitment) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Sep. 30, 2013
Dec. 31, 2014
Dec. 31, 2013
Unfunded Commitments [Line Items]      
Reserve for unfunded commitments   $ 0.8cob_LoansAndLeasesReceivableUnfundedCommitmentsReserve $ 0.5cob_LoansAndLeasesReceivableUnfundedCommitmentsReserve
Letters of credit [Member]      
Unfunded Commitments [Line Items]      
Expected usage percentage of remaining unfunded commitments 10.00%cob_LoansAndLeasesReceivableUnfundedCommitmentsExpectedUsagePercentage
/ us-gaap_FinancialInstrumentAxis
= us-gaap_LetterOfCreditMember