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Intangible Assets (Details Textual) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Goodwill and Intangible Assets Disclosure [Abstract]      
Amortization of core deposit premiums $ 1,408,000us-gaap_AmortizationOfIntangibleAssets $ 1,407,000us-gaap_AmortizationOfIntangibleAssets $ 1,407,000us-gaap_AmortizationOfIntangibleAssets
Unpaid principal balance of mortgage loans serviced for others (unconsolidated) 235,000,000cob_ServicingAssetsUnconsolidatedLoansServicedUnpaidPrincipalBalance 190,200,000cob_ServicingAssetsUnconsolidatedLoansServicedUnpaidPrincipalBalance  
Mortgage servicing rights, fair value $ 1,700,000us-gaap_ServicingAssetAtFairValueAmount $ 1,600,000us-gaap_ServicingAssetAtFairValueAmount