Securities - Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 7,353,180	$ 7,323,980
Gross Unrealized Gains	294,578	143,872
Gross Unrealized Losses	(8,307)	(20,490)
Fair Value	7,639,451	7,447,362
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,549	63,835
Gross Unrealized Gains	1,056	408
Gross Unrealized Losses		(165)
Fair Value	31,605	64,078
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	94,195	89,867
Gross Unrealized Gains	6,421	3,154
Fair Value	100,616	93,021
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,067,786	4,030,688
Gross Unrealized Gains	171,441	58,184
Gross Unrealized Losses	(4,578)	(17,078)
Fair Value	4,234,649	4,071,794
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,980,676	2,954,276
Gross Unrealized Gains	114,292	78,867
Gross Unrealized Losses	(1,276)	(3,226)
Fair Value	3,093,692	3,029,917
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	179,974	185,314
Gross Unrealized Gains	1,368	3,259
Gross Unrealized Losses	(2,453)	(21)
Fair Value	$ 178,889	$ 188,552