Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Gross Unrealized Losses
Less Than Twelve Months	$ (80)	$ (962)
Twelve Months or More	(37,945)	(72,420)
Total	(38,025)	(73,382)
Estimated Fair Value
Less Than Twelve Months	51,991	237,384
Twelve Months or More	2,561,520	2,740,272
Total	2,613,511	2,977,656
Gross Unrealized Losses
Less than twelve months	0	(3)
Over twelve months	(784)	(3,505)
Total	(784)	(3,508)
Estimated Fair Value
Less than twelve months	0	2,059
Over twelve months	30,640	169,177
Total	30,640	171,236
Obligations of state and political subdivisions
Gross Unrealized Losses
Less Than Twelve Months	0	(9)
Twelve Months or More	(32)	(714)
Total	(32)	(723)
Estimated Fair Value
Less Than Twelve Months	0	4,112
Twelve Months or More	11,925	30,268
Total	11,925	34,380
Gross Unrealized Losses
Less than twelve months	0	(3)
Over twelve months	(42)	(2,494)
Total	(42)	(2,497)
Estimated Fair Value
Less than twelve months	0	2,059
Over twelve months	13,160	151,699
Total	13,160	153,758
Mortgage-backed securities: Residential agency
Gross Unrealized Losses
Less Than Twelve Months	(71)	(816)
Twelve Months or More	(29,692)	(56,257)
Total	(29,763)	(57,073)
Estimated Fair Value
Less Than Twelve Months	40,608	197,057
Twelve Months or More	2,081,552	2,193,862
Total	2,122,160	2,390,919
Gross Unrealized Losses
Less than twelve months	0	0
Over twelve months	(742)	(1,011)
Total	(742)	(1,011)
Estimated Fair Value
Less than twelve months	0	0
Over twelve months	17,480	17,478
Total	17,480	17,478
Mortgage-backed securities: Commercial agency
Gross Unrealized Losses
Less Than Twelve Months	(5)	(43)
Twelve Months or More	(7,711)	(14,756)
Total	(7,716)	(14,799)
Estimated Fair Value
Less Than Twelve Months	7,001	18,190
Twelve Months or More	434,500	483,565
Total	441,501	501,755
Other securities
Gross Unrealized Losses
Less Than Twelve Months	(4)	(94)
Twelve Months or More	(510)	(693)
Total	(514)	(787)
Estimated Fair Value
Less Than Twelve Months	4,382	18,025
Twelve Months or More	33,543	32,577
Total	$ 37,925	$ 50,602