Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 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	69	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	21	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	(643)	(29)
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	66,800
Securities Sold under Agreements to Repurchase	$ 66,540	89,182
Transfers between hierarchy levels	0
Agency securities	$ 70,292	98,516
Non-Agency Securities, at Fair Value	552	579
Financial Instruments, Owned, Other, at Fair Value	574	976
U.S. Treasury securities	3,721	97
TBA securities	(664)	(190)
Derivative Liability	138	6
Debt of consolidated variable interest entities, at fair value	214	228
Obligation to return securities borrowed under reverse repurchase agreements, at fair value	4,886	9,543
Interest rate swaps	664	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	3,721	111
Total liabilities	4,955	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	72,440	100,618
Total liabilities	283	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	$ 69	$ 4