Derivative and Other Hedging Instruments (Effect Of Derivative Instruments Not Designated As Hedges On Comprehensive Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	$ (951)	$ (562)
Gain (loss) on REIT Equity Securities		2
Gain Loss on Other Debt	(5)
Interest Only And Principal Only Strip [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Unrealized Gain (Loss) on Securities	11	11
Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Additions	0	0
Settlement, Expirations or Exercise	400	1,600
Amount Gain/(Loss) Recognized in Income on Derivatives	(7)	(17)
TBA and Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	7,295	14,412
Additions	17,959	45,500
Settlement, Expirations or Exercise		(55,039)
Notional Amount	5,813	4,873
Amount Gain/(Loss) Recognized in Income on Derivatives	216	234
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Settlement, Expirations or Exercise	(19,441)
Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	40,525
Additions	(1,000)	(3,500)
Settlement, Expirations or Exercise	3,350	2,275
Notional Amount	38,175
Amount Gain/(Loss) Recognized in Income on Derivatives	(1,005)	(746)
Receiver Swaption [Member] | Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Additions		0
Settlement, Expirations or Exercise		(3,500)
Amount Gain/(Loss) Recognized in Income on Derivatives		17
Future [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(1,860)	(730)
Additions	(1,860)	(730)
Settlement, Expirations or Exercise	1,860	730
Notional Amount	(1,860)	(730)
Amount Gain/(Loss) Recognized in Income on Derivatives	(77)	(20)
Other derivative instruments [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	12
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(2,150)	(6,800)
Notional Amount	(1,750)	(5,200)
Not Designated as Hedging Instrument [Member] | Receiver Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount		4,250
Notional Amount		750
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(40,525)	(43,700)
Notional Amount	(38,175)	(44,925)
Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(1,714)	(5,392)
Additions	(1,980)	(4,173)
Settlement, Expirations or Exercise	559	6,212
Notional Amount	(3,135)	(3,353)
Amount Gain/(Loss) Recognized in Income on Derivatives	(83)	(82)
Long [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Trading Securities	0
Long [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	5	52
Trading Securities		4,261	$ 25	$ 2,411
Trading Securities Added During the Period	180	15,562
Notional Amount Of Trading Securities Settlement Expiration During The Period	$ (205)	$ (13,712)