Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	154	196
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (1)	$ (29,116)
12 Months or More Unrealized Losses	(142,161)	(93,742)
Total Unrealized Losses	(142,162)	(122,858)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	172	2,119,812
12 Months or More Fair Value	3,619,406	3,052,744
Total Fair Value	3,619,578	5,172,556
U.S. Treasury securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(994)
12 Months or More Unrealized Losses		(11,127)
Total Unrealized Losses		(12,121)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		48,182
12 Months or More Fair Value		344,073
Total Fair Value		392,255
Government-sponsored enterprises debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(642)
12 Months or More Unrealized Losses	(6,778)	(6,469)
Total Unrealized Losses	(6,778)	(7,111)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		59,358
12 Months or More Fair Value	157,939	183,243
Total Fair Value	157,939	242,601
Government agency mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(976)
12 Months or More Unrealized Losses	(15,174)	(4,492)
Total Unrealized Losses	(15,174)	(5,468)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		200,963
12 Months or More Fair Value	373,891	150,427
Total Fair Value	373,891	351,390
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1)	(1)
12 Months or More Unrealized Losses	(5,293)	(4,129)
Total Unrealized Losses	(5,294)	(4,130)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	172	63
12 Months or More Fair Value	125,869	168,342
Total Fair Value	126,041	168,405
Collateralized mortgage obligations: Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(23,236)
12 Months or More Unrealized Losses	(97,171)	(53,510)
Total Unrealized Losses	(97,171)	(76,746)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		1,473,170
12 Months or More Fair Value	2,475,532	1,803,338
Total Fair Value	2,475,532	3,276,508
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(3,203)
12 Months or More Unrealized Losses	(17,745)	(14,015)
Total Unrealized Losses	(17,745)	(17,218)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		327,435
12 Months or More Fair Value	486,175	403,321
Total Fair Value	$ 486,175	730,756
States and political subdivisions
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(64)
Total Unrealized Losses		(64)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		10,641
Total Fair Value		$ 10,641