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Investment Securities - Investment Available for Sale (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 97,569	$ 77,984
Allowance for Credit Losses	(1)
Gross Unrealized Gains	2,997	1,561
Gross Unrealized Losses	(120)	(332)
Fair Value	100,445	79,213
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,302	4,122
Allowance for Credit Losses	0
Gross Unrealized Gains	16	6
Gross Unrealized Losses	0	(4)
Fair Value	9,318	4,124
RMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,248	62,003
Allowance for Credit Losses	0
Gross Unrealized Gains	2,326	1,120
Gross Unrealized Losses	(108)	(284)
Fair Value	75,466	62,839
RMBS, Non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,035	1,235
Allowance for Credit Losses	(1)
Gross Unrealized Gains	204	266
Gross Unrealized Losses	(1)	(2)
Fair Value	1,237	1,499
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	74,283	63,238
Allowance for Credit Losses	(1)
Gross Unrealized Gains	2,530	1,386
Gross Unrealized Losses	(109)	(286)
Fair Value	76,703	64,338
CMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,298	9,303
Allowance for Credit Losses	0
Gross Unrealized Gains	448	165
Gross Unrealized Losses	(11)	(42)
Fair Value	11,735	9,426
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,686	1,321
Allowance for Credit Losses	0
Gross Unrealized Gains	3	4
Gross Unrealized Losses	0	0
Fair Value	2,689	1,325
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 1,800	$ 117

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