Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 14,868,761	$ 15,487,442
Unrealized Gains	231,218	17,040
Unrealized Losses	(35,017)	(316,246)
Fair Value	15,064,962	15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	585,817	640,990
Unrealized Gains	14,111	2,148
Unrealized Losses	0	(4,850)
Fair Value	599,928	638,288
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7,706,133	8,103,638
Unrealized Gains	105,024	5,011
Unrealized Losses	(7,790)	(157,767)
Fair Value	7,803,367	7,950,882
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,912,839	2,183,204
Unrealized Gains	1,780	0
Unrealized Losses	(14,322)	(62,272)
Fair Value	1,900,297	2,120,932
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	197,962	214,483
Unrealized Gains	109	608
Unrealized Losses	(357)	(14)
Fair Value	197,714	215,077
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,886,958	2,769,706
Unrealized Gains	49,057	6,969
Unrealized Losses	(12,455)	(64,374)
Fair Value	2,923,560	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,579,052	1,575,421
Unrealized Gains	61,137	2,304
Unrealized Losses	(93)	(26,969)
Fair Value	$ 1,640,096	$ 1,550,756