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Investment Securities - Investment Available for Sale (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 97,802 | $ 97,569
Allowance for Credit Losses | (2) | (1)
Gross Unrealized Gains | 2,204 | 2,997
Gross Unrealized Losses | (839) | (120)
Fair Value | 99,165 | 100,445
Accrued interest receivable | 237 | 230
U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 9,222 | 9,302
Allowance for Credit Losses | 0 | 0
Gross Unrealized Gains | 26 | 16
Gross Unrealized Losses | 0 | 0
Fair Value | 9,248 | 9,318
RMBS, Agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 73,579 | 73,248
Allowance for Credit Losses | 0 | 0
Gross Unrealized Gains | 1,649 | 2,326
Gross Unrealized Losses | (783) | (108)
Fair Value | 74,445 | 75,466
RMBS, Non-agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 973 | 1,035
Allowance for Credit Losses | (2) | (1)
Gross Unrealized Gains | 206 | 204
Gross Unrealized Losses | 0 | (1)
Fair Value | 1,177 | 1,237
RMBS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 74,552 | 74,283
Allowance for Credit Losses | (2) | (1)
Gross Unrealized Gains | 1,855 | 2,530
Gross Unrealized Losses | (783) | (109)
Fair Value | 75,622 | 76,703
CMBS, Agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 11,015 | 11,298
Allowance for Credit Losses | 0 | 0
Gross Unrealized Gains | 315 | 448
Gross Unrealized Losses | (56) | (11)
Fair Value | 11,274 | 11,735
Other securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,013 | 2,686
Allowance for Credit Losses | 0 | 0
Gross Unrealized Gains | 8 | 3
Gross Unrealized Losses | 0 | 0
Fair Value | 3,021 | 2,689
Asset-backed Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fair Value | $ 2,100 | $ 1,800

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