Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Future [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
TBA securities	$ 0	$ 14
Derivative Liability	3	0
Interest Rate Swaption [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	6	126
Interest Rate Swap [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	0	21
Interest Rate Swap [Member] \| Derivative liabilities, at fair value [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	0	2
Purchases Of TBAs And Forward Settling Agency Securities [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| TBA and Forward Settling Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
TBA securities	(130)	(29)
Securities Sold under Agreements to Repurchase	$ 69,685	89,182
Transfers between hierarchy levels	no
Agency securities	$ 75,488	98,516
Non-Agency Securities, at Fair Value	599	579
Financial Instruments, Owned, Other, at Fair Value	712	976
U.S. Treasury securities	1,181	97
TBA securities	(140)	(190)
Derivative Liability	3	6
Debt of consolidated variable interest entities, at fair value	204	228
Interest rate swaps	140	190
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,181	111
Total liabilities	7,932	9,543
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	77,283	100,618
Total liabilities	204	234
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Sale Of TBA And Forward Settling Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ 0	$ 4

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Fair value of firm holdings in securities classified as other. Includes pledged and unpledged holdings.

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The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Fair value of financial and nonfinancial obligations.

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Amount of long-term debt classified as other.

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