Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 12,044,717	$ 13,894,348
Unrealized Gains	603,367	140,713
Unrealized Losses	(20)	(20,142)
Carrying Value	12,648,064	14,014,919
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,745,238	6,815,874
Unrealized Gains	298,602	82,267
Unrealized Losses	0	(4,131)
Carrying Value	4,043,840	6,894,010
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	100,000	100,000
Unrealized Gains	1,687	0
Unrealized Losses	0	(453)
Carrying Value	101,687	99,547
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,169	9,037
Unrealized Gains	1	1
Unrealized Losses	(18)	0
Carrying Value	22,152	9,038
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,758,167	4,109,372
Unrealized Gains	185,347	39,438
Unrealized Losses	(2)	(19)
Carrying Value	4,943,512	4,148,791
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,409,673	1,520,414
Unrealized Gains	56,002	17,929
Unrealized Losses	0	0
Carrying Value	1,465,675	1,538,343
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,009,470	1,339,651
Unrealized Gains	61,728	1,078
Unrealized Losses	0	(15,539)
Carrying Value	$ 2,071,198	$ 1,325,190