Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,927,254	$ 3,327,805	$ 3,182,772
Securities available-for-sale, Gross Unrealized Holding Gains	191,779	87,346	80,341
Securities available-for-sale, Gross Unrealized Holding Losses	(170)	(1,834)	(3,621)
Securities available-for-sale, Estimated Fair Value	4,118,863	3,413,317	3,259,492
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,083	9,997	9,984
Securities available-for-sale, Gross Unrealized Holding Gains	40	22	36
Securities available-for-sale, Gross Unrealized Holding Losses	(1)
Securities available-for-sale, Estimated Fair Value	10,122	10,019	10,020
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,836,572	1,231,619	1,103,682
Securities available-for-sale, Gross Unrealized Holding Gains	113,757	57,764	53,593
Securities available-for-sale, Gross Unrealized Holding Losses	(50)	(400)	(61)
Securities available-for-sale, Estimated Fair Value	1,950,279	1,288,983	1,157,214
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	4,398	4,643	4,643
Securities available-for-sale, Gross Unrealized Holding Gains	171	65	51
Securities available-for-sale, Estimated Fair Value	4,569	4,708	4,694
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,517,139	1,586,872	1,579,544
Securities available-for-sale, Gross Unrealized Holding Gains	54,652	23,139	19,732
Securities available-for-sale, Gross Unrealized Holding Losses	(118)	(1,148)	(3,101)
Securities available-for-sale, Estimated Fair Value	1,571,673	1,608,863	1,596,175
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	559,062	494,674	484,919
Securities available-for-sale, Gross Unrealized Holding Gains	23,159	6,356	6,929
Securities available-for-sale, Gross Unrealized Holding Losses	(1)	(286)	(459)
Securities available-for-sale, Estimated Fair Value	$ 582,220	$ 500,744	$ 491,389

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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