Derivative and Other Hedging Instruments (Fair Value Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative asset, fair value	$ 664		$ 190
Derivative liability, fair value	(138)		(6)
Derivative assets, at fair value	(664)		(190)
Derivative Liability	(138)		(6)
U.S. Treasury securities	3,721		97
U.S. Treasury Securities - short	(4,886)		(9,543)
Total - (short)/long, net	(1,165)		(9,446)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(3,180)	$ (996)
TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	37,750	18,442
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period		(18,772)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	693	83
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	46,475		79,075
Derivative liability, fair value	0		(2)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	49,975	5,350
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(82,575)	(8,800)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(2,795)	(596)
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative asset, fair value	0		21
Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	9,550		8,850
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	2,000	0
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(1,300)	(950)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(134)	(27)
Interest Rate Swaption [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative asset, fair value	21		126
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative liability, fair value	(69)		(4)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(24,793)
US Treasury Securities [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(104)	(31)
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		14
Derivative Liability	(69)		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	(643)		(29)
Short [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	6,045	4,770
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	11,024	(7,380)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(937)	(425)
Long [Member] | US Treasury Securities [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	97	0
Debt Securities, Trading, and Equity Securities, FV-NI	3,569	120	$ 95	$ 45
Trading Securities Added During the Period	6,461	405
Notional Amount Of Trading Securities Settlement Expiration During The Period	$ (2,987)	$ (330)