Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Gross Unrealized Losses
Less than twelve months	$ (70,865)	$ (27,757)
Over twelve months	(56,298)	(36,084)
Total	(127,163)	(63,841)
Estimated Fair Value
Less than twelve months	3,033,959	2,813,985
Over twelve months	1,267,402	1,304,515
Total	4,301,361	4,118,500
Gross Unrealized Losses
Less than twelve months	(4,077)	(265)
Over twelve months	(1,057)	(660)
Total	(5,134)	(925)
Estimated Fair Value
Less than twelve months	150,511	66,150
Over twelve months	21,163	22,300
Total	171,674	88,450
US Treasury Securities
Gross Unrealized Losses
Less than twelve months	0
Over twelve months	0
Total	0
Estimated Fair Value
Less than twelve months	693
Over twelve months	0
Total	693
U.S. Government-Sponsored Enterprise Obligations
Gross Unrealized Losses
Less than twelve months	(419)	(254)
Over twelve months	(235)	(152)
Total	(654)	(406)
Estimated Fair Value
Less than twelve months	29,580	29,744
Over twelve months	9,765	9,848
Total	39,345	39,592
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(1,147)	(326)
Over twelve months	(3,009)	(1,167)
Total	(4,156)	(1,493)
Estimated Fair Value
Less than twelve months	96,138	31,601
Over twelve months	66,404	68,609
Total	162,542	100,210
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(68,161)	(26,263)
Over twelve months	(53,054)	(34,765)
Total	(121,215)	(61,028)
Estimated Fair Value
Less than twelve months	2,847,244	2,677,338
Over twelve months	1,191,233	1,226,058
Total	4,038,477	3,903,396
Other Securities
Gross Unrealized Losses
Less than twelve months	(1,138)	(914)
Over twelve months	0	0
Total	(1,138)	(914)
Estimated Fair Value
Less than twelve months	60,304	75,302
Over twelve months	0	0
Total	60,304	75,302
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(4,066)	(263)
Over twelve months	(54)	(12)
Total	(4,120)	(275)
Estimated Fair Value
Less than twelve months	150,188	65,817
Over twelve months	2,976	3,031
Total	153,164	68,848
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(11)	(2)
Over twelve months	(1,003)	(648)
Total	(1,014)	(650)
Estimated Fair Value
Less than twelve months	323	333
Over twelve months	18,187	19,269
Total	$ 18,510	$ 19,602