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Investment Securities - Investment Available for Sale (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 96,735	$ 77,984
Allowance for Credit Losses	(3)
Gross Unrealized Gains	3,218	1,561
Gross Unrealized Losses	(97)	(332)
Fair Value	99,853	79,213
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,331	4,122
Allowance for Credit Losses	0
Gross Unrealized Gains	14	6
Gross Unrealized Losses	0	(4)
Fair Value	9,345	4,124
RMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,948	62,003
Allowance for Credit Losses	0
Gross Unrealized Gains	2,506	1,120
Gross Unrealized Losses	(88)	(284)
Fair Value	75,366	62,839
RMBS, Non-agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,093	1,235
Allowance for Credit Losses	(3)
Gross Unrealized Gains	204	266
Gross Unrealized Losses	(1)	(2)
Fair Value	1,293	1,499
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	74,041	63,238
Allowance for Credit Losses	(3)
Gross Unrealized Gains	2,710	1,386
Gross Unrealized Losses	(89)	(286)
Fair Value	76,659	64,338
CMBS, Agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,953	9,303
Allowance for Credit Losses	0
Gross Unrealized Gains	490	165
Gross Unrealized Losses	(8)	(42)
Fair Value	11,435	9,426
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,410	1,321
Allowance for Credit Losses	0
Gross Unrealized Gains	4	4
Gross Unrealized Losses	0	0
Fair Value	2,414	1,325
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 1,500	$ 117

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