Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 6,767,967	$ 6,447,510	$ 4,961,438
Securities available-for-sale, Gross Unrealized Holding Gains	29,691	149,380	166,558
Securities available-for-sale, Gross Unrealized Holding Losses	(295,163)	(23,711)	(18,365)
Securities available-for-sale, Estimated Fair Value	6,502,495	6,573,179	5,109,631
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	311,860	126,716
Securities available-for-sale, Gross Unrealized Holding Gains		125
Securities available-for-sale, Gross Unrealized Holding Losses	(10,516)
Securities available-for-sale, Estimated Fair Value	301,344	126,841
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	2,590,308	2,638,369	2,418,227
Securities available-for-sale, Gross Unrealized Holding Gains	26,628	116,319	106,988
Securities available-for-sale, Gross Unrealized Holding Losses	(87,455)	(1,217)	(8,198)
Securities available-for-sale, Estimated Fair Value	2,529,481	2,753,471	2,517,017
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	3,450,006	3,256,746	2,073,647
Securities available-for-sale, Gross Unrealized Holding Gains	2,139	23,990	43,575
Securities available-for-sale, Gross Unrealized Holding Losses	(186,954)	(21,287)	(8,628)
Securities available-for-sale, Estimated Fair Value	3,265,191	3,259,449	2,108,594
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	341,506	356,207	431,137
Securities available-for-sale, Gross Unrealized Holding Gains	924	8,914	15,916
Securities available-for-sale, Gross Unrealized Holding Losses	(4,752)	(1)
Securities available-for-sale, Estimated Fair Value	337,678	365,120	447,053
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	74,287	69,472	38,427
Securities available-for-sale, Gross Unrealized Holding Gains		32	79
Securities available-for-sale, Gross Unrealized Holding Losses	(5,486)	(1,206)	(1,539)
Securities available-for-sale, Estimated Fair Value	$ 68,801	$ 68,298	$ 36,967

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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Amount, before tax, of unrealized loss 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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