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Derivative and Other Hedging Instruments (Fair Value Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative asset, fair value	$ 140		$ 140			$ 190
Derivative liability, fair value	(3)		(3)			(6)
Derivative assets, at fair value	(140)		(140)			(190)
Derivative Liability	(3)		(3)			(6)
U.S. Treasury securities	1,181		1,181			97
U.S. Treasury Securities - short	(7,929)		(7,929)			(9,543)
Total - (short)/long, net	(6,748)		(6,748)			(9,446)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(237)	$ (1,437)	(3,417)	$ (2,433)
Other derivative asset	4		4
TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	75,186	27,859	112,936	46,301
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period				(42,410)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	220	163	913	246
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	42,075		42,075			79,075
Derivative liability, fair value	0		0			(2)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	25,750	65,000	75,725	70,350
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(30,150)	(38,225)	(112,725)	(47,025)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(379)	(1,019)	(3,174)	(1,615)
Interest Rate Swap [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative asset, fair value	0		0			21
Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	9,350		9,350			8,850
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	0	2,650	2,000	2,650
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(200)	(800)	(1,500)	(1,750)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(14)	(25)	(148)	(52)
Interest Rate Swaption [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative asset, fair value	6		6			126
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative liability, fair value	0		0			(4)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(75,705)	(23,638)	(100,498)
US Treasury Securities [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(8)	(57)	(112)	(88)
US Treasury Securities [Member] \| Fair Value, Recurring [Member] \| Future [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative [Line Items]
Derivative assets, at fair value	0		0			14
Derivative Liability	(3)		(3)			0
TBA and Forward Settling Agency Securities [Member] \| Fair Value, Recurring [Member] \| Purchases Of TBAs And Forward Settling Agency Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative assets, at fair value	(130)		(130)			(29)
Short [Member] \| US Treasury Securities [Member]
Derivative [Line Items]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	4,187	2,536	10,232	7,306
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	1,185	14,026	12,209	(21,406)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(60)	(505)	(997)	(930)
Long [Member] \| US Treasury Securities [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	4	6	101	6
Debt Securities, Trading, and Equity Securities, FV-NI	1,132	1,134	1,132	1,134	$ 3,569	$ 95	$ 120	$ 45
Trading Securities Added During the Period	550	1,018	7,011	1,423
Notional Amount Of Trading Securities Settlement Expiration During The Period	$ (2,987)	$ (4)	$ (5,974)	$ (334)