Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 15,055,255	$ 15,487,442
Unrealized Gains	91,211	17,040
Unrealized Losses	(149,958)	(316,246)
Fair Value	14,996,508	15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	585,892	640,990
Unrealized Gains	2,858	2,148
Unrealized Losses	(2,950)	(4,850)
Fair Value	585,800	638,288
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7,887,579	8,103,638
Unrealized Gains	41,279	5,011
Unrealized Losses	(50,917)	(157,767)
Fair Value	7,877,941	7,950,882
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,058,789	2,183,204
Unrealized Gains	66	0
Unrealized Losses	(40,414)	(62,272)
Fair Value	2,018,441	2,120,932
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	206,421	214,483
Unrealized Gains	114	608
Unrealized Losses	(727)	(14)
Fair Value	205,808	215,077
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,745,662	2,769,706
Unrealized Gains	16,866	6,969
Unrealized Losses	(53,676)	(64,374)
Fair Value	2,708,852	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,570,912	1,575,421
Unrealized Gains	30,028	2,304
Unrealized Losses	(1,274)	(26,969)
Fair Value	$ 1,599,666	$ 1,550,756