Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Gross Unrealized Losses
Less Than Twelve Months	$ (1,918)	$ (962)
Twelve Months or More	(2,296)	(72,420)
Total	(4,214)	(73,382)
Estimated Fair Value
Less Than Twelve Months	469,662	237,384
Twelve Months or More	315,192	2,740,272
Total	784,854	2,977,656
Gross Unrealized Losses
Less than twelve months	(2)	(3)
Over twelve months	(194)	(3,505)
Total	(196)	(3,508)
Estimated Fair Value
Less than twelve months	924	2,059
Over twelve months	9,551	169,177
Total	10,475	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	(1,829)	(816)
Twelve Months or More	(2,101)	(56,257)
Total	(3,930)	(57,073)
Estimated Fair Value
Less Than Twelve Months	447,333	197,057
Twelve Months or More	299,396	2,193,862
Total	746,729	2,390,919
Gross Unrealized Losses
Less than twelve months	(2)	0
Over twelve months	(194)	(1,011)
Total	(196)	(1,011)
Estimated Fair Value
Less than twelve months	924	0
Over twelve months	9,551	17,478
Total	10,475	17,478
Mortgage-backed securities: Commercial agency
Gross Unrealized Losses
Less Than Twelve Months	(83)	(43)
Twelve Months or More	(96)	(14,756)
Total	(179)	(14,799)
Estimated Fair Value
Less Than Twelve Months	8,961	18,190
Twelve Months or More	10,994	483,565
Total	19,955	501,755
Other securities
Gross Unrealized Losses
Less Than Twelve Months	(6)	(94)
Twelve Months or More	(99)	(693)
Total	(105)	(787)
Estimated Fair Value
Less Than Twelve Months	13,368	18,025
Twelve Months or More	4,802	32,577
Total	$ 18,170	$ 50,602