Borrowed Funds - Schedule of Long-Term Borrowings (Detail) - Long-term borrowings - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
FHLB advances, net	$ 789,185	$ 1,592,252
Subordinated debt, net	638,259	403,413
Securities sold under agreements to repurchase	0	300,000
Total long-term borrowings	1,427,444	2,295,665
Unamortized prepayment penalties and other purchase accounting adjustments		2,600
Deferred issuance costs	$ 6,100	$ 2,700