Other Borrowed Funds and Subordinated Debentures - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2004	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
FHLBB advances		$ 40,000,000	$ 40,000,000	$ 40,000,000
Subordinated debt securities issued		36,083,000	36,083,000
Prior Restructuring FHLBB [Member]
Debt Instrument [Line Items]
FHLBB advances			$ 15,000,000
FHLBB Weighted average rate			3.33%
FHLBB Weighted average maturity			14 months
Post Restructuring FHLBB [Member]
Debt Instrument [Line Items]
FHLBB Weighted average rate			2.37%
FHLBB Weighted average maturity			60 months
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		591,974,000
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
LIBOR rate		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		2.09%	3.76%
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%