Other Borrowed Funds and Subordinated Debentures - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2004	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
FHLBB advances		$ 45,000,000	$ 55,000,000	$ 35,000,000
Subordinated debt securities issued		36,083,000	36,083,000
Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Subordinated debt securities issued	$ 36,083,000
Subordinated debt securities due year	2034
Federal Home Loan Bank Borrowings [Member]
Debt Instrument [Line Items]
Bank's remaining term borrowing capacity at the FHLBB		239,163,000
Line of credit with the FHLBB		$ 14,500,000
Subordinated Debt [Member] | Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Liquidation value of shares of cumulative trust preferred securities		$ 1,000
Period of dividend		10 years
LIBOR rate		Three-month LIBOR rate plus 1.87% for the remaining 20 years
LIBOR rate trust preferred securities		1.87%
Duration of LIBOR rate		20 years
Investment coupon rate		2.83%
Other Borrowed Funds [Member]
Debt Instrument [Line Items]
Federal funds purchased		$ 0	$ 0
Cumulative Preferred Stock Subject to Mandatory Redemption [Member] | Subordinated Debt [Member] | Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Shares of cumulative trust preferred securities		35,000
Trust preferred securities annual dividend rate		6.65%