Derivative and Other Hedging Instruments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2014
Forward Contracts [Member] | Interest Rate Swap [Member]
Average Maturity (Years)			6 years 9 months 9 days		5 years 6 months 7 days
Interest Rate Swaption [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ (1)	$ (23)	$ (12)	$ (27)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	0	0	0	500
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(350)	(1,800)	(1,450)	(3,650)
TBA and Forward Settling Agency Securities [Member]
Notional Amount	14,881	7,128	14,881	7,128	$ 7,295	$ 6,756	$ 6,941	$ 14,412
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	67	213	391	337
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	37,881	33,403	75,906	97,270
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period		(33,216)		(104,554)
US Treasury Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	15	(23)	(106)	(28)
Gain Loss on Other Debt	(2)	7	(8)	16
Unrealized gain on derivative instruments, net	7	24	38	79
Net periodic interest costs on swaps	58	131	247	369
Derivative Instruments, Gain Reclassified from Accumulated OCI into Income, Effective Portion	51	107	209	290
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	249	(795)	(1,070)	(1,116)
Gain (loss) on REIT Equity Securities				(4)
Discontinuation of Election to Account for Interest Rate Swaps as Designated Cash Flow Hedges [Member]
Net Unrealized Gain (Loss) on Swaps	1	61	1	61	39	8	85	140
Interest Only And Principal Only Strip [Member]
Unrealized Gain (Loss) on Securities	(6)	10	5	14
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Notional Amount	700	3,650	700	3,650	2,150	1,050	5,450	6,800
Short [Member] | US Treasury Securities [Member]
Notional Amount	5,356	1,301	5,356	1,301	1,714	2,930	2,250	5,392
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	14	(25)	(142)	(89)
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	(2,696)	4,335	5,329	10,732
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	270	(5,284)	(1,687)	(14,823)
Long [Member] | US Treasury Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	1	29	7	(35)
Trading Securities Added During the Period	90	4,704	495	31,915
Notional Amount Of Trading Securities Settlement Expiration During The Period	(107)	(9,121)	(475)	(33,551)
Trading Securities		$ 775		$ 775	$ 25	$ 62	$ 5,192	$ 2,411
Long [Member]
Trading Securities	45		45
Other Derivatives [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 1		$ 15