Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Gross Unrealized Losses
Less Than Twelve Months	$ (2,278)	$ (962)
Twelve Months or More	(2,195)	(72,420)
Total	(4,473)	(73,382)
Estimated Fair Value
Less Than Twelve Months	511,891	237,384
Twelve Months or More	283,176	2,740,272
Total	795,067	2,977,656
Gross Unrealized Losses
Less than twelve months	(52)	(3)
Over twelve months	(187)	(3,505)
Total	(239)	(3,508)
Estimated Fair Value
Less than twelve months	6,308	2,059
Over twelve months	9,074	169,177
Total	15,382	171,236
Obligations of state and political subdivisions
Gross Unrealized Losses
Less Than Twelve Months	0	(9)
Twelve Months or More	0	(714)
Total	0	(723)
Estimated Fair Value
Less Than Twelve Months	590	4,112
Twelve Months or More	0	30,268
Total	590	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
Residential agency
Gross Unrealized Losses
Less Than Twelve Months	(1,893)	(816)
Twelve Months or More	(1,943)	(56,257)
Total	(3,836)	(57,073)
Estimated Fair Value
Less Than Twelve Months	441,332	197,057
Twelve Months or More	268,383	2,193,862
Total	709,715	2,390,919
Gross Unrealized Losses
Less than twelve months	(52)	0
Over twelve months	(187)	(1,011)
Total	(239)	(1,011)
Estimated Fair Value
Less than twelve months	6,308	0
Over twelve months	9,074	17,478
Total	15,382	17,478
Commercial agency
Gross Unrealized Losses
Less Than Twelve Months	(252)	(43)
Twelve Months or More	(152)	(14,756)
Total	(404)	(14,799)
Estimated Fair Value
Less Than Twelve Months	56,728	18,190
Twelve Months or More	10,301	483,565
Total	67,029	501,755
Other securities
Gross Unrealized Losses
Less Than Twelve Months	(133)	(94)
Twelve Months or More	(100)	(693)
Total	(233)	(787)
Estimated Fair Value
Less Than Twelve Months	13,241	18,025
Twelve Months or More	4,492	32,577
Total	$ 17,733	$ 50,602