Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 14,407,078	$ 15,487,442
Unrealized Gains	310,417	17,040
Unrealized Losses	(18,693)	(316,246)
Fair Value	14,698,802	15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	518,841	640,990
Unrealized Gains	10,528	2,148
Unrealized Losses	(22)	(4,850)
Fair Value	529,347	638,288
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7,376,458	8,103,638
Unrealized Gains	135,996	5,011
Unrealized Losses	(4,190)	(157,767)
Fair Value	7,508,264	7,950,882
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,754,498	2,183,204
Unrealized Gains	2,254	0
Unrealized Losses	(10,094)	(62,272)
Fair Value	1,746,658	2,120,932
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	188,120	214,483
Unrealized Gains	105	608
Unrealized Losses	(373)	(14)
Fair Value	187,852	215,077
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,826,344	2,769,706
Unrealized Gains	75,422	6,969
Unrealized Losses	(3,046)	(64,374)
Fair Value	2,898,720	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,742,817	1,575,421
Unrealized Gains	86,112	2,304
Unrealized Losses	(968)	(26,969)
Fair Value	$ 1,827,961	$ 1,550,756