Derivative and Other Hedging Instruments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) month	Dec. 31, 2017 USD ($) month	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Interest Rate Swaption [Member]
Average Maturity (Years)	7 years 4 months 24 days	8 years 10 months 24 days
Cash Flow Hedges Derivative Instruments Not Designated As Hedging Instruments Average Months To Expiration | month	6	10
Notional Amount	$ 3,500	$ 6,650
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	90	(66)	$ (3)
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, Average Fixed Interest Rate	2.93%	2.72%
Interest Rate Derivatives, at Fair Value, Net	$ 37	$ 75
Options At Cost	98	159
TBA and Forward Settling Agency Securities [Member]
Notional Amount	7,152	15,474	10,916	$ 7,295
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(299)	330	(59)
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)
US Treasury Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	48	0	(5)
Unrealized gain on derivative instruments, net	0	0	39
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(181)	191	(323)
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Notional Amount	3,500
Short [Member] | US Treasury Securities [Member]
Notional Amount	21,345	10,699	8,061	1,714
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(161)	(141)	7
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)
Long [Member] | US Treasury Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	1	1	134
Trading Securities Added During the Period	1,949	404	961
Notional Amount Of Trading Securities Settlement Expiration During The Period	(1,904)	(593)	(797)
Debt Securities, Trading, and Equity Securities, FV-NI		$ 0	$ 189	$ 25
Long [Member]
Debt Securities, Trading, and Equity Securities, FV-NI	$ 45