Securities - Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 8,116,473	$ 7,323,980
Gross Unrealized Gains	367,908	143,872
Gross Unrealized Losses	(757)	(20,490)
Fair Value	8,483,624	7,447,362
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,904	63,835
Gross Unrealized Gains	1,016	408
Gross Unrealized Losses		(165)
Fair Value	30,920	64,078
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	413,537	89,867
Gross Unrealized Gains	7,065	3,154
Gross Unrealized Losses	(50)
Fair Value	420,552	93,021
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,549,567	4,030,688
Gross Unrealized Gains	200,261	58,184
Gross Unrealized Losses	(371)	(17,078)
Fair Value	4,749,457	4,071,794
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,066,713	2,954,276
Gross Unrealized Gains	156,257	78,867
Gross Unrealized Losses	(336)	(3,226)
Fair Value	3,222,634	3,029,917
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	56,752	185,314
Gross Unrealized Gains	3,309	3,259
Gross Unrealized Losses		(21)
Fair Value	$ 60,061	$ 188,552