Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Gross Unrealized Losses
Less Than Twelve Months	$ 0	$ (962)
Twelve Months or More	(7,563)	(72,420)
Total	(7,563)	(73,382)
Estimated Fair Value
Less Than Twelve Months	0	237,384
Twelve Months or More	1,153,560	2,740,272
Total	1,153,560	2,977,656
Gross Unrealized Losses
Less than twelve months	0	(3)
Over twelve months	(342)	(3,505)
Total	(342)	(3,508)
Estimated Fair Value
Less than twelve months	0	2,059
Over twelve months	17,039	169,177
Total	17,039	171,236
Obligations of state and political subdivisions
Gross Unrealized Losses
Less Than Twelve Months		(9)
Twelve Months or More		(714)
Total		(723)
Estimated Fair Value
Less Than Twelve Months		4,112
Twelve Months or More		30,268
Total		34,380
Gross Unrealized Losses
Less than twelve months		(3)
Over twelve months		(2,494)
Total		(2,497)
Estimated Fair Value
Less than twelve months		2,059
Over twelve months		151,699
Total		153,758
Mortgage-backed securities: Residential agency
Gross Unrealized Losses
Less Than Twelve Months	0	(816)
Twelve Months or More	(6,364)	(56,257)
Total	(6,364)	(57,073)
Estimated Fair Value
Less Than Twelve Months	0	197,057
Twelve Months or More	1,026,095	2,193,862
Total	1,026,095	2,390,919
Gross Unrealized Losses
Less than twelve months	0	0
Over twelve months	(342)	(1,011)
Total	(342)	(1,011)
Estimated Fair Value
Less than twelve months	0	0
Over twelve months	17,039	17,478
Total	17,039	17,478
Mortgage-backed securities: Commercial agency
Gross Unrealized Losses
Less Than Twelve Months	0	(43)
Twelve Months or More	(1,107)	(14,756)
Total	(1,107)	(14,799)
Estimated Fair Value
Less Than Twelve Months	0	18,190
Twelve Months or More	122,450	483,565
Total	122,450	501,755
Other securities
Gross Unrealized Losses
Less Than Twelve Months	0	(94)
Twelve Months or More	(92)	(693)
Total	(92)	(787)
Estimated Fair Value
Less Than Twelve Months	0	18,025
Twelve Months or More	5,015	32,577
Total	$ 5,015	$ 50,602