Investment Securities - Investment Available for Sale (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 96,989	$ 97,569
Allowance for Credit Losses	(1)	(1)
Gross Unrealized Gains	1,819	2,997
Gross Unrealized Losses	(658)	(120)
Fair Value	98,149	100,445
Accrued interest receivable	227	230
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,094	9,302
Allowance for Credit Losses	0	0
Gross Unrealized Gains	15	16
Gross Unrealized Losses	0	0
Fair Value	9,109	9,318
RMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,924	73,248
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,325	2,326
Gross Unrealized Losses	(617)	(108)
Fair Value	74,632	75,466
RMBS, Non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	844	1,035
Allowance for Credit Losses	(1)	(1)
Gross Unrealized Gains	206	204
Gross Unrealized Losses	0	(1)
Fair Value	1,049	1,237
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	74,768	74,283
Allowance for Credit Losses	(1)	(1)
Gross Unrealized Gains	1,531	2,530
Gross Unrealized Losses	(617)	(109)
Fair Value	75,681	76,703
CMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,973	11,298
Allowance for Credit Losses	0	0
Gross Unrealized Gains	265	448
Gross Unrealized Losses	(41)	(11)
Fair Value	10,197	11,735
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,154	2,686
Allowance for Credit Losses	0	0
Gross Unrealized Gains	8	3
Gross Unrealized Losses	0	0
Fair Value	3,162	2,689
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 2,300	$ 1,800

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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