Borrowings - Additional Information (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Maximum borrowing capacity with FHLB	$ 2,400,000,000
Outstanding short-term letters of credit	937,600,000	$ 669,700,000
Available borrowing capacity from the Federal Home Loan Bank	1,400,000,000
Investment securities collateral for Federal Reserve Bank Discount Window Lending program	31,500,000	40,700,000
Outstanding amount of federal fund line of credit with Federal Reserve Bank of Philadelphia	0	0
Amount of maintained line of credit with correspondent bank - parent company	10,000,000	10,000,000
Outstanding amount of line of credit with correspondent bank - parent company	0	0
Total committed borrowing capacity	2,400,000,000	2,200,000,000
Total committed borrowing capacity available	1,400,000,000	1,500,000,000
Amount of maintained federal fund lines of credit with correspondent banks	$ 400,000,000	$ 460,000,000