Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 22,919,000	$ 30,245,000
Unrealized Gains	286,000	683,000
Unrealized Losses	(221,000)	(15,000)
Carrying value	22,984,000	30,913,000
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,122,000	4,198,000
Unrealized Gains	170,000	272,000
Unrealized Losses	(15,000)	0
Carrying value	9,277,000	4,470,000
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	212,000	234,000
Unrealized Gains	0	4,000
Unrealized Losses	(4,000)	(1,000)
Carrying value	208,000	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	10,902,000	13,271,000
Unrealized Gains	69,000	233,000
Unrealized Losses	(187,000)	(1,000)
Carrying value	10,784,000	13,503,000
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,129,000	8,077,000
Unrealized Gains	11,000	40,000
Unrealized Losses	(2,000)	(10,000)
Carrying value	1,138,000	8,107,000
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,539,000	4,441,000
Unrealized Gains	36,000	134,000
Unrealized Losses	(13,000)	(3,000)
Carrying value	$ 1,562,000	$ 4,572,000