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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
 0 Months Ended 12 Months Ended
Dec. 30, 2005
Dec. 31, 2014
Reporting_Unit
Segment
Dec. 31, 2013
Dec. 31, 2012
Summary Of Significant Accounting Policies And Other Information [Line Items]        
Equity ownership interest   100.00%us-gaap_MinorityInterestOwnershipPercentageByParent    
Number of reportable segments   1us-gaap_NumberOfReportableSegments    
Highly liquid assets, original maturity period   Three months or less    
Highly liquid certificates of deposit, original maturity period   More than 90 days but less than one year    
Securities held for trading   $ 0us-gaap_TradingSecurities    
Impairment recognized   $ 0cnbka_ImpairmentRecognized    
Loans discontinued delinquency period   90 days    
Loan to value ratio   80.00%cnbka_MaximumLoanToValueRatio    
Number of reporting units   1us-gaap_NumberOfReportingUnits    
Goodwill impairment description   Goodwill impairment is evaluated by first assessing qualitative factors (events and circumstances) to determine whether it is more likely than not (meaning a likelihood of more than 50 percent) that the fair value of a reporting unit is less than its carrying amount. If, after considering all relevant events and circumstances, an entity determines it is not more likely than not that the fair value of a reporting unit is less than its carrying amount, then performing the two-step impairment test will be unnecessary.    
Nonqualified stock options   85.00%cnbka_NonqualifiedStockOptionsPercent    
Options to purchase an aggregate of shares     20,375us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber 23,350us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Time period over which stock option exercisable $ 31.60us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice      
Average price of common stock $ 29.28cnbka_AveragePriceOfCommonStock      
Stock option granted   0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross  
Minimum [Member]        
Summary Of Significant Accounting Policies And Other Information [Line Items]        
Supplemental plan   1 year    
Maximum [Member]        
Summary Of Significant Accounting Policies And Other Information [Line Items]        
Time period over which stock option exercisable   10 years    
Class A Common Stock [Member]        
Summary Of Significant Accounting Policies And Other Information [Line Items]        
Approved stock option plan for purchase of common stock   150,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		    
Options to purchase an aggregate of shares 23,950us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember