Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 23,776,000	$ 30,245,000
Unrealized Gains	330,000	683,000
Unrealized Losses	(230,000)	(15,000)
Carrying Value	23,876,000	30,913,000
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,051,000	4,198,000
Unrealized Gains	189,000	272,000
Unrealized Losses	(5,000)	0
Carrying Value	6,235,000	4,470,000
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	84,000	234,000
Unrealized Gains	0	4,000
Unrealized Losses	(3,000)	(1,000)
Carrying Value	81,000	237,000
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,000	24,000
Unrealized Gains	0	0
Unrealized Losses	0	0
Carrying Value	15,000	24,000
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,606,000	13,271,000
Unrealized Gains	80,000	233,000
Unrealized Losses	(124,000)	(1,000)
Carrying Value	11,562,000	13,503,000
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,497,000	8,077,000
Unrealized Gains	17,000	40,000
Unrealized Losses	(90,000)	(10,000)
Carrying Value	4,424,000	8,107,000
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,523,000	4,441,000
Unrealized Gains	44,000	134,000
Unrealized Losses	(8,000)	(3,000)
Carrying Value	$ 1,559,000	$ 4,572,000