Securities - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, allowance for credit loss, excluding accrued interest	$ 0		$ 0
Debt securities, available-for-sale, accrued interest, after allowance for credit loss	14,200,000		14,200,000		$ 14,300,000
Debt securities, available-for-sale, accrued interest writeoff	0		0
Gross realized gains from sales of available for sale securities (less than for the three months ended June 20, 2021)	0	$ 100,000	1,000,000	$ 1,200,000
Gross realized losses from sales of available for sale securities	(100,000)	$ 0	(3,300,000)	0
Other-than-temporary impairment				$ 0
Held-to-maturity debt securities, fair value	451,747,000		451,747,000		0
Debt securities, held-to-maturity, allowance for credit loss	0		0
Debt securities, held-to-maturity, accrued interest, after allowance for credit loss	1,000,000		1,000,000
Debt securities, held-to-maturity, accrued interest, writeoff	0		0
Deposit liabilities, collateral issued, financial instruments	481,700,000		481,700,000		2,200,000,000
Century Bancorp, Inc.
Debt Securities, Available-for-sale [Line Items]
Deposit liabilities, collateral issued, financial instruments	$ 453,700,000		$ 453,700,000		$ 2,100,000,000