Other Borrowed Funds and Subordinated Debentures - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2004	Dec. 31, 2012	Jan. 10, 2005
Debt Instrument [Line Items]
FHLBB advances	$ 35,000,000	$ 35,000,000		$ 35,000,000
Restructuring of FHLBB advances		14,500,000
Weighted average rate of restructured FHLBB advances		3.16%
Weighted average remaining maturity period of FHLB		12 months
Weighted average rate subsequent to restructured of FHLBB advances		3.24%
Weighted average remaining maturity subsequent to Restructured FHLBB advances		68 months
Subordinated debt securities issued	36,083,000	36,083,000
Subordinated debentures issued for new Unconsolidated subsidiary	29,639,000
Subordinated debt securities due year	2029
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	221,384,000
Line of credit with the FHLBB	14,500,000
Subordinated Debt [Member]
Debt Instrument [Line Items]
Liquidation value of shares of cumulative trust preferred securities	$ 10
Percentage of preferred trust securities					8.30%
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
Other Borrowed Funds [Member]
Debt Instrument [Line Items]
Federal funds purchased	0	0
Outstanding loan	$ 0	$ 144,000
Cumulative Preferred Stock Subject to Mandatory Redemption [Member] | Subordinated Debt [Member]
Debt Instrument [Line Items]
Shares of cumulative trust preferred securities	2,875,000
Trust preferred securities annual dividend rate	8.30%
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%