Securities - Summary Of Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 4,023,719	$ 3,125,184
Unrealized Gains	39,929	62,549
Unrealized Losses	(77,395)	(3,872)
Fair Value	3,986,253	3,183,861
Government-sponsored residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,713,898	2,106,658
Unrealized Gains	23,199	42,142
Unrealized Losses	(45,395)	0
Fair Value	2,691,702	2,148,800
Government-sponsored commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	119,064	17,054
Unrealized Gains	0	27
Unrealized Losses	(2,793)	0
Fair Value	116,271	17,081
U.S. Agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	860,554	670,468
Unrealized Gains	0	113
Unrealized Losses	(28,882)	(3,872)
Fair Value	831,672	666,709
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	69,385	70,106
Unrealized Gains	30	263
Unrealized Losses	(325)	0
Fair Value	69,090	70,369
State and municipal bonds and obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	260,818	260,898
Unrealized Gains	16,700	20,004
Unrealized Losses	0	0
Fair Value	$ 277,518	$ 280,902