Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Gross Unrealized Losses
Less than twelve months	$ (82,592)	$ (27,757)
Over twelve months	(65,856)	(36,084)
Total	(148,448)	(63,841)
Estimated Fair Value
Less than twelve months	3,041,866	2,813,985
Over twelve months	1,320,200	1,304,515
Total	4,362,066	4,118,500
Gross Unrealized Losses
Less than twelve months	(3,943)	(265)
Over twelve months	(1,379)	(660)
Total	(5,322)	(925)
Estimated Fair Value
Less than twelve months	147,353	66,150
Over twelve months	24,601	22,300
Total	171,954	88,450
U.S. Government-Sponsored Enterprise Obligations
Gross Unrealized Losses
Less than twelve months	(445)	(254)
Over twelve months	(270)	(152)
Total	(715)	(406)
Estimated Fair Value
Less than twelve months	29,554	29,744
Over twelve months	9,730	9,848
Total	39,284	39,592
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(991)	(326)
Over twelve months	(2,927)	(1,167)
Total	(3,918)	(1,493)
Estimated Fair Value
Less than twelve months	77,872	31,601
Over twelve months	66,467	68,609
Total	144,339	100,210
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(79,939)	(26,263)
Over twelve months	(61,631)	(34,765)
Total	(141,570)	(61,028)
Estimated Fair Value
Less than twelve months	2,847,301	2,677,338
Over twelve months	1,220,866	1,226,058
Total	4,068,167	3,903,396
Other Securities
Gross Unrealized Losses
Less than twelve months	(1,217)	(914)
Over twelve months	(1,028)	0
Total	(2,245)	(914)
Estimated Fair Value
Less than twelve months	87,139	75,302
Over twelve months	23,137	0
Total	110,276	75,302
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(3,930)	(263)
Over twelve months	(188)	(12)
Total	(4,118)	(275)
Estimated Fair Value
Less than twelve months	147,033	65,817
Over twelve months	6,861	3,031
Total	153,894	68,848
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(13)	(2)
Over twelve months	(1,191)	(648)
Total	(1,204)	(650)
Estimated Fair Value
Less than twelve months	320	333
Over twelve months	17,740	19,269
Total	$ 18,060	$ 19,602