Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Amortized cost	$ 3,857,213	$ 3,486,274
Gross unrealized gain	8,781	7,374
Gross unrealized losses	(42,041)	(47,551)
Estimated fair value	3,823,953	3,446,097
Schedule of Held-to-maturity Securities
Amortized cost	86,018	89,216
Gross unrealized gains	1,737	1,666
Gross unrealized losses	(942)	(950)
Estimated fair value	86,813	89,932
Fair value of securities held as collateral	1,600,000	1,500,000
U.S. Government-Sponsored Enterprise Obligations
Schedule of Available-for-sale Securities
Amortized cost	212,430	212,662
Gross unrealized gain	199	245
Gross unrealized losses	(463)	(549)
Estimated fair value	212,166	212,358
Obligations of State and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized cost	297,712	286,458
Gross unrealized gain	3,190	1,948
Gross unrealized losses	(3,812)	(5,207)
Estimated fair value	297,090	283,199
Schedule of Held-to-maturity Securities
Amortized cost	62,315	64,726
Gross unrealized gains	1,682	1,609
Gross unrealized losses	(99)	(133)
Estimated fair value	63,898	66,202
Mortgage-Backed Securities
Schedule of Available-for-sale Securities
Amortized cost	3,238,530	2,888,180
Gross unrealized gain	4,937	4,820
Gross unrealized losses	(37,505)	(41,291)
Estimated fair value	3,205,962	2,851,709
Schedule of Held-to-maturity Securities
Amortized cost	23,703	24,490
Gross unrealized gains	55	57
Gross unrealized losses	(843)	(817)
Estimated fair value	22,915	23,730
Other Securities
Schedule of Available-for-sale Securities
Amortized cost	108,541	98,974
Gross unrealized gain	455	361
Gross unrealized losses	(261)	(504)
Estimated fair value	$ 108,735	$ 98,831