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Derivative and Other Hedging Instruments (Fair Value Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative asset, fair value	$ 273	$ 205
Derivative liability, fair value	(84)	(28)
Derivative assets, at fair value	273	205
Derivative Liability	(84)	(28)
U.S. Treasury securities	46	0
U.S. Treasury Securities - short	(21,431)	(10,467)
Total - (short)/long, net	(21,385)	(10,467)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(181)	191	$ (323)
TBA and Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	7,152	15,474	10,916	$ 7,295
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	194,534	237,601	116,439
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period		(233,043)	(112,818)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(299)	330	(59)
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	51,625	43,700
Derivative liability, fair value	0	(1)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	14,350	14,825	15,650
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(6,425)	(8,300)	(19,000)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	140	67	(397)
Interest Rate Swap [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative asset, fair value	126	81
Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	3,500	6,650
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	1,250	6,450	500
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(4,400)	(1,000)	(1,450)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	90	(66)	(3)
Interest Rate Swaption [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative asset, fair value	37	75
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Derivative [Line Items]
Derivative liability, fair value	(40)	(27)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(202,856)
US Treasury Securities [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	48	0	(5)
US Treasury Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Future [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative [Line Items]
Derivative assets, at fair value	0	19
Derivative Liability	44	0
TBA and Forward Settling Agency Securities [Member] \| Fair Value, Measurements, 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	110	30
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	51,625
Not Designated as Hedging Instrument [Member] \| Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	3,500
Short [Member] \| US Treasury Securities [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	21,345	10,699	8,061	1,714
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	(19,278)	14,030	9,884
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	8,632	(11,392)	(3,537)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(161)	(141)	7
Long [Member]
Derivative [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	45
Long [Member] \| US Treasury Securities [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	1	1	134
Debt Securities, Trading, and Equity Securities, FV-NI		0	189	$ 25
Trading Securities Added During the Period	1,949	404	961
Notional Amount Of Trading Securities Settlement Expiration During The Period	$ (1,904)	$ (593)	$ (797)