Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	$ 2,742,482	$ 1,916,658
Available-for-sale, Gross Unrealized Gains	50,154	63,237
Available-for-sale, Gross Unrealized Losses	(17,424)	(1,759)
Available-for-sale, Estimated Fair Value	2,775,212	1,978,136
Held-to-maturity, Amortized Cost	813,740	731,212
Held-to-maturity, Gross Unrealized Gains	29,948	37,030
Held-to-maturity, Gross Unrealized Losses	(3,816)	(59)
Held-to-maturity securities, Fair value	839,872	768,183
Total securities, Amortized Cost	3,556,222	2,647,870
Total securities, Gross Unrealized Gains	80,102	100,267
Total securities, Gross Unrealized Losses	(21,240)	(1,818)
Total securities, Estimated Fair Value	3,615,084	2,746,319
US Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	29,986	39,975
Available-for-sale, Gross Unrealized Gains	11	7
Available-for-sale, Estimated Fair Value	29,997	39,982
U.S. Government Sponsored Entities and Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	174,472	204,109
Available-for-sale, Gross Unrealized Gains	4,564	7,715
Available-for-sale, Gross Unrealized Losses	(2,706)	(142)
Available-for-sale, Estimated Fair Value	176,330	211,682
Held-to-maturity, Amortized Cost	7,611	7,779
Held-to-maturity, Gross Unrealized Gains	106	265
Held-to-maturity securities, Fair value	7,717	8,044
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	2,123,853	1,230,106
Available-for-sale, Gross Unrealized Gains	30,901	35,979
Available-for-sale, Gross Unrealized Losses	(13,961)	(1,348)
Available-for-sale, Estimated Fair Value	2,140,793	1,264,737
Held-to-maturity, Amortized Cost	80,376	89,151
Held-to-maturity, Gross Unrealized Gains	2,738	3,251
Held-to-maturity, Gross Unrealized Losses	(5)
Held-to-maturity securities, Fair value	83,109	92,402
Commercial Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	284,440	308,903
Available-for-sale, Gross Unrealized Gains	7,962	11,464
Available-for-sale, Gross Unrealized Losses	(721)	(269)
Available-for-sale, Estimated Fair Value	291,681	320,098
Obligations of State and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	105,778	108,602
Available-for-sale, Gross Unrealized Gains	6,056	7,160
Available-for-sale, Gross Unrealized Losses	(36)
Available-for-sale, Estimated Fair Value	111,798	115,762
Held-to-maturity, Amortized Cost	692,616	601,128
Held-to-maturity, Gross Unrealized Gains	24,501	30,173
Held-to-maturity, Gross Unrealized Losses	(3,811)	(59)
Held-to-maturity securities, Fair value	713,306	631,242
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	23,953	24,963
Available-for-sale, Gross Unrealized Gains	660	912
Available-for-sale, Estimated Fair Value	24,613	25,875
Held-to-maturity, Amortized Cost	33,137	33,154
Held-to-maturity, Gross Unrealized Gains	2,603	3,341
Held-to-maturity securities, Fair value	$ 35,740	$ 36,495

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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