Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	$ 2,928,495	$ 1,916,658
Available-for-sale, Gross Unrealized Gains	49,376	63,237
Available-for-sale, Gross Unrealized Losses	(13,607)	(1,759)
Available-for-sale, Estimated Fair Value	2,964,264	1,978,136
Held-to-maturity, Amortized Cost	902,172	731,212
Held-to-maturity, Gross Unrealized Gains	33,442	37,030
Held-to-maturity, Gross Unrealized Losses	(1,127)	(59)
Held-to-maturity securities, Fair value	934,487	768,183
Total securities, Amortized Cost	3,830,667	2,647,870
Total securities, Gross Unrealized Gains	82,818	100,267
Total securities, Gross Unrealized Losses	(14,734)	(1,818)
Total securities, Estimated Fair Value	3,898,751	2,746,319
US Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	19,995	39,975
Available-for-sale, Gross Unrealized Gains	3	7
Available-for-sale, Estimated Fair Value	19,998	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	207,004	204,109
Available-for-sale, Gross Unrealized Gains	6,653	7,715
Available-for-sale, Gross Unrealized Losses	(1,167)	(142)
Available-for-sale, Estimated Fair Value	212,490	211,682
Held-to-maturity, Amortized Cost	6,867	7,779
Held-to-maturity, Gross Unrealized Gains	216	265
Held-to-maturity securities, Fair value	7,083	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	2,264,266	1,230,106
Available-for-sale, Gross Unrealized Gains	28,106	35,979
Available-for-sale, Gross Unrealized Losses	(11,606)	(1,348)
Available-for-sale, Estimated Fair Value	2,280,766	1,264,737
Held-to-maturity, Amortized Cost	70,745	89,151
Held-to-maturity, Gross Unrealized Gains	2,365	3,251
Held-to-maturity, Gross Unrealized Losses	(5)
Held-to-maturity securities, Fair value	73,105	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	309,525	308,903
Available-for-sale, Gross Unrealized Gains	7,586	11,464
Available-for-sale, Gross Unrealized Losses	(826)	(269)
Available-for-sale, Estimated Fair Value	316,285	320,098
Obligations of State and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	103,763	108,602
Available-for-sale, Gross Unrealized Gains	6,342	7,160
Available-for-sale, Gross Unrealized Losses	(8)
Available-for-sale, Estimated Fair Value	110,097	115,762
Held-to-maturity, Amortized Cost	791,441	601,128
Held-to-maturity, Gross Unrealized Gains	28,212	30,173
Held-to-maturity, Gross Unrealized Losses	(1,122)	(59)
Held-to-maturity securities, Fair value	818,531	631,242
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	23,942	24,963
Available-for-sale, Gross Unrealized Gains	686	912
Available-for-sale, Estimated Fair Value	24,628	25,875
Held-to-maturity, Amortized Cost	33,119	33,154
Held-to-maturity, Gross Unrealized Gains	2,649	3,341
Held-to-maturity securities, Fair value	$ 35,768	$ 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, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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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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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Securities amortized cost.

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