Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities
Amortized cost	$ 98,330	$ 116,960
Gross unrealized gains	2,981	3,392
Gross unrealized losses	(700)	(871)
Estimated fair value	100,611	119,481
Schedule of Available-for-sale Securities
Amortized cost	2,801,751	2,147,276
Gross Unrealized Gains	30,096	22,296
Gross Unrealized Losses	(4,042)	(10,719)
Securities available for sale	2,827,805	2,158,853
U.S. Government-Sponsored Enterprise Obligations
Schedule of Held-to-maturity Securities
Amortized cost		10,000
Gross unrealized gains		88
Gross unrealized losses		0
Estimated fair value		10,088
Schedule of Available-for-sale Securities
Amortized cost	277,822	317,386
Gross Unrealized Gains	2,743	1,700
Gross Unrealized Losses	(45)	(3,533)
Securities available for sale	280,520	315,553
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities
Amortized cost	73,821	77,597
Gross unrealized gains	2,878	3,153
Gross unrealized losses	(74)	(145)
Estimated fair value	76,625	80,605
Schedule of Available-for-sale Securities
Amortized cost	181,626	86,513
Gross Unrealized Gains	4,502	3,679
Gross Unrealized Losses	0	(2)
Securities available for sale	186,128	90,190
Mortgage-Backed Securities
Schedule of Held-to-maturity Securities
Amortized cost	24,509	29,363
Gross unrealized gains	103	151
Gross unrealized losses	(626)	(726)
Estimated fair value	23,986	28,788
Schedule of Available-for-sale Securities
Amortized cost	2,246,002	1,741,917
Gross Unrealized Gains	22,157	16,882
Gross Unrealized Losses	(3,647)	(7,184)
Securities available for sale	2,264,512	1,751,615
Other Securities
Schedule of Available-for-sale Securities
Amortized cost	96,301	1,460
Gross Unrealized Gains	694	35
Gross Unrealized Losses	(350)	0
Securities available for sale	$ 96,645	$ 1,495