Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,913,534	$ 6,670,090
Gross Unrealized Gains	56,055	13,980
Gross Unrealized Losses	(77,720)	(141,270)
Fair Value	6,891,869	6,542,800
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	259,000	248,494
Gross Unrealized Gains	595	192
Gross Unrealized Losses	(1,149)	(1,556)
Fair Value	258,446	247,130
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	90,096	200
Gross Unrealized Gains	1,347
Gross Unrealized Losses	(6)	(1)
Fair Value	91,437	199
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,962,630	3,914,289
Gross Unrealized Gains	23,294	6,145
Gross Unrealized Losses	(67,139)	(108,223)
Fair Value	3,918,785	3,812,211
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,557,727	2,507,107
Gross Unrealized Gains	30,622	7,643
Gross Unrealized Losses	(9,392)	(31,490)
Fair Value	2,578,957	$ 2,483,260
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,081
Gross Unrealized Gains	197
Gross Unrealized Losses	(34)
Fair Value	$ 44,244