Securities - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 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	13,600,000		13,600,000		$ 14,300,000
Debt securities, available-for-sale, accrued interest writeoff	0		0
Other-than-temporary impairment loss, debt securities, available-for-sale		$ 0		$ 0
Held-to-maturity debt securities, fair value	431,477,000		431,477,000		0
Debt securities, held-to-maturity, allowance for credit loss, excluding accrued interest	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	438,800,000		438,800,000		2,200,000,000
Century Bancorp, Inc.
Debt Securities, Available-for-sale [Line Items]
Deposit liabilities, collateral issued, financial instruments	$ 422,300,000		$ 422,300,000		$ 2,100,000,000