Borrowings - Additional Information (Detail) - USD ($)		9 Months Ended
	Aug. 05, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Maximum borrowing capacity with FHLB		$ 2,100,000,000
Outstanding short-term letters of credit		650,800,000		$ 535,600,000
Available borrowing capacity from the Federal Home Loan Bank		1,200,000,000
Investment securities collateral for Federal Reserve Bank Discount Window Lending program		69,500,000		94,800,000
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		10,000,000.0		10,000,000.0
Outstanding amount of line of credit with correspondent bank - parent company		0		0
Total committed borrowing capacity		2,200,000,000		1,900,000,000
Total committed borrowing capacity available		1,300,000,000		1,200,000,000
Amount of maintained federal fund lines of credit with correspondent banks		460,000,000.0		$ 504,000,000.0
Proceeds from issuance of subordinated notes		98,448,000	$ 0
Paycheck Protection Program Lending Facility [Member]
Debt Instrument [Line Items]
PPP loans pledged to FRB		0
Securities Sold under Agreements to Repurchase [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Long-term debt with variable interest rate		$ 5,000,000.0
Subordinated Debt [Member]
Debt Instrument [Line Items]
Aggregate principal amount of debt	$ 100,000,000.0
Annual fixed rate of interest	5.00%
Proceeds from issuance of subordinated notes	$ 98,400,000
Debt issuance costs	$ 1,600,000
Term of debt issuance costs amortization	5 years
Subordinated Debt [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate of subordinated debt		495.20%
Term of variable rate basis for subordinated notes		3 months