Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 4,961,438	$ 4,177,179	$ 3,950,510
Securities available-for-sale, Gross Unrealized Holding Gains	166,558	216,048	159,486
Securities available-for-sale, Gross Unrealized Holding Losses	(18,365)	(198)	(2,927)
Securities available-for-sale, Estimated Fair Value	5,109,631	4,393,029	4,107,069
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total			10,039
Securities available-for-sale, Gross Unrealized Holding Gains			74
Securities available-for-sale, Estimated Fair Value			10,113
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	2,418,227	2,283,616	1,731,392
Securities available-for-sale, Gross Unrealized Holding Gains	106,988	143,339	78,996
Securities available-for-sale, Gross Unrealized Holding Losses	(8,198)	(79)	(2,717)
Securities available-for-sale, Estimated Fair Value	2,517,017	2,426,876	1,807,671
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	38,427	4,398	4,624
Securities available-for-sale, Gross Unrealized Holding Gains	79	159	133
Securities available-for-sale, Gross Unrealized Holding Losses	(1,539)		(6)
Securities available-for-sale, Estimated Fair Value	36,967	4,557	4,751
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	2,073,647	1,421,922	1,617,341
Securities available-for-sale, Gross Unrealized Holding Gains	43,575	50,473	62,659
Securities available-for-sale, Gross Unrealized Holding Losses	(8,628)	(115)	(20)
Securities available-for-sale, Estimated Fair Value	2,108,594	1,472,280	1,679,980
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	431,137	467,243	587,114
Securities available-for-sale, Gross Unrealized Holding Gains	15,916	22,077	17,624
Securities available-for-sale, Gross Unrealized Holding Losses		(4)	(184)
Securities available-for-sale, Estimated Fair Value	$ 447,053	$ 489,316	$ 604,554

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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