Borrowed Funds - Schedule of Long-Term Borrowings (Detail) - Long-term borrowings - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
FHLB advances, net	$ 2,148,633	$ 1,480,012
Net subordinated note carrying value	403,936	292,414
Securities sold under agreements to repurchase	300,000	350,000
Total long-term borrowings	2,852,569	2,122,426
Unamortized prepayment penalties and other purchase accounting adjustments	503	2,800
Deferred issuance costs	$ 2,800	$ 1,200