Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 6,546,471	$ 6,447,510	$ 5,981,374
Securities available-for-sale, Gross Unrealized Holding Gains	531	149,380	153,068
Securities available-for-sale, Gross Unrealized Holding Losses	(801,559)	(23,711)	(14,458)
Securities available-for-sale, Estimated Fair Value	5,745,443	6,573,179	6,119,984
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	504,457	126,716
Securities available-for-sale, Gross Unrealized Holding Gains	2	125
Securities available-for-sale, Gross Unrealized Holding Losses	(27,283)	0
Securities available-for-sale, Estimated Fair Value	477,176	126,841
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	2,368,605	2,638,369	2,568,224
Securities available-for-sale, Gross Unrealized Holding Gains	514	116,319	103,646
Securities available-for-sale, Gross Unrealized Holding Losses	(285,307)	(1,217)	(6,503)
Securities available-for-sale, Estimated Fair Value	2,083,812	2,753,471	2,665,367
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	3,153,864	3,256,746	2,999,369
Securities available-for-sale, Gross Unrealized Holding Gains	14	23,990	35,950
Securities available-for-sale, Gross Unrealized Holding Losses	(456,997)	(21,287)	(7,170)
Securities available-for-sale, Estimated Fair Value	2,696,881	3,259,449	3,028,149
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	406,530	356,207	375,614
Securities available-for-sale, Gross Unrealized Holding Gains	1	8,914	13,402
Securities available-for-sale, Gross Unrealized Holding Losses	(19,185)	(1)	0
Securities available-for-sale, Estimated Fair Value	387,346	365,120	389,016
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	113,015	69,472	38,167
Securities available-for-sale, Gross Unrealized Holding Gains	0	32	70
Securities available-for-sale, Gross Unrealized Holding Losses	(12,787)	(1,206)	(785)
Securities available-for-sale, Estimated Fair Value	$ 100,228	$ 68,298	$ 37,452

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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