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Securities Sold under Agreements to Repurchase - Summary of Securities Sold Under Agreements to Repurchase (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Banking and Thrift [Abstract]      
Amount outstanding at December 31 $ 212,360us-gaap_SecuritiesSoldUnderAgreementsToRepurchase $ 214,440us-gaap_SecuritiesSoldUnderAgreementsToRepurchase $ 191,390us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Weighted average rate at December 31 0.18%cnbka_SecuritiesSoldUnderAgreementToRepurchaseWeightedAverageRate 0.18%cnbka_SecuritiesSoldUnderAgreementToRepurchaseWeightedAverageRate 0.17%cnbka_SecuritiesSoldUnderAgreementToRepurchaseWeightedAverageRate
Maximum amount outstanding at any month end 243,750cnbka_SecuritiesSoldUnderAgreementsToRepurchaseMaximumOutstanding 214,440cnbka_SecuritiesSoldUnderAgreementsToRepurchaseMaximumOutstanding 213,730cnbka_SecuritiesSoldUnderAgreementsToRepurchaseMaximumOutstanding
Daily average balance outstanding during the year $ 216,937cnbka_SecuritiesSoldUnderAgreementsToRepurchaseDailyAverageOutstandingBalance $ 203,888cnbka_SecuritiesSoldUnderAgreementsToRepurchaseDailyAverageOutstandingBalance $ 174,624cnbka_SecuritiesSoldUnderAgreementsToRepurchaseDailyAverageOutstandingBalance
Weighted average rate during the year 0.18%cnbka_SecuritiesSoldUnderAgreementToRepurchaseWeightedAverageRateDuringPeriod 0.18%cnbka_SecuritiesSoldUnderAgreementToRepurchaseWeightedAverageRateDuringPeriod 0.21%cnbka_SecuritiesSoldUnderAgreementToRepurchaseWeightedAverageRateDuringPeriod