Securities - Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 8,887,734	$ 7,323,980
Gross Unrealized Gains	413,665	143,872
Gross Unrealized Losses	(1,711)	(20,490)
Fair Value	9,299,688	7,447,362
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,911	63,835
Gross Unrealized Gains	829	408
Gross Unrealized Losses		(165)
Fair Value	30,740	64,078
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,554	89,867
Gross Unrealized Gains	6,395	3,154
Fair Value	95,949	93,021
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,266,394	4,030,688
Gross Unrealized Gains	202,944	58,184
Gross Unrealized Losses	(1,157)	(17,078)
Fair Value	5,468,181	4,071,794
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,424,309	2,954,276
Gross Unrealized Gains	199,848	78,867
Gross Unrealized Losses	(538)	(3,226)
Fair Value	3,623,619	3,029,917
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,566	185,314
Gross Unrealized Gains	3,649	3,259
Gross Unrealized Losses	(16)	(21)
Fair Value	$ 81,199	$ 188,552