Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 26,159,161	$ 30,244,896
Unrealized Gains	325,659	682,530
Unrealized Losses	(498,349)	(14,988)
Carrying Value	25,986,471	30,912,438
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,244,175	4,197,858
Unrealized Gains	195,444	271,977
Unrealized Losses	(1,223)	(107)
Carrying Value	4,438,396	4,469,728
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	230,375	233,727
Unrealized Gains	1,155	4,165
Unrealized Losses	(5,435)	(585)
Carrying Value	226,095	237,307
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,454	24,491
Unrealized Gains	0	1
Unrealized Losses	(4)	0
Carrying Value	23,450	24,492
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,663,089	13,271,482
Unrealized Gains	85,210	232,850
Unrealized Losses	(233,388)	(651)
Carrying Value	12,514,911	13,503,681
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,471,566	8,076,832
Unrealized Gains	19,567	40,010
Unrealized Losses	(234,507)	(10,278)
Carrying Value	7,256,626	8,106,564
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,526,502	4,440,506
Unrealized Gains	24,283	133,527
Unrealized Losses	(23,792)	(3,367)
Carrying Value	$ 1,526,993	$ 4,570,666