Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Amortized cost	$ 4,755,104	$ 3,486,274
Gross unrealized gain	13,326	7,374
Gross unrealized losses	(32,091)	(47,551)
Estimated fair value	4,736,339	3,446,097
Schedule of Held-to-maturity Securities
Amortized cost	175,906	89,216
Gross unrealized gains	1,460	1,666
Gross unrealized losses	(1,051)	(950)
Estimated fair value	176,315	89,932
Fair value of securities held as collateral	1,900,000	1,500,000
U.S. Government-Sponsored Enterprise Obligations
Schedule of Available-for-sale Securities
Amortized cost	60,940	212,662
Gross unrealized gain	99	245
Gross unrealized losses	(171)	(549)
Estimated fair value	60,868	212,358
Obligations of State and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized cost	273,938	286,458
Gross unrealized gain	5,226	1,948
Gross unrealized losses	(1,516)	(5,207)
Estimated fair value	277,648	283,199
Schedule of Held-to-maturity Securities
Amortized cost	153,775	64,726
Gross unrealized gains	1,413	1,609
Gross unrealized losses	(483)	(133)
Estimated fair value	154,705	66,202
Mortgage-Backed Securities
Schedule of Available-for-sale Securities
Amortized cost	4,300,673	2,888,180
Gross unrealized gain	7,448	4,820
Gross unrealized losses	(29,968)	(41,291)
Estimated fair value	4,278,153	2,851,709
Schedule of Held-to-maturity Securities
Amortized cost	22,131	24,490
Gross unrealized gains	47	57
Gross unrealized losses	(568)	(817)
Estimated fair value	21,610	23,730
Other Securities
Schedule of Available-for-sale Securities
Amortized cost	119,553	98,974
Gross unrealized gain	553	361
Gross unrealized losses	(436)	(504)
Estimated fair value	$ 119,670	$ 98,831