Derivative and Other Hedging Instruments (Effect Of Derivative Instruments Not Designated As Hedges On Comprehensive Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	$ 249	$ (795)	$ (1,070)	$ (1,116)
Gain (loss) on REIT Equity Securities				(4)
Gain Loss on Other Debt	(2)	7	(8)	16
Interest Only And Principal Only Strip [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Unrealized Gain (Loss) on Securities	(6)	10	5	14
Receiver Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives				4
Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Additions	0	0	0	(500)
Settlement, Expirations or Exercise	350	1,800	1,450	3,650
Amount Gain/(Loss) Recognized in Income on Derivatives	(1)	(23)	(12)	(27)
TBA and Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(6,756)	(6,941)	(7,295)	(14,412)
Additions	(37,881)	(33,403)	(75,906)	(97,270)
Settlement, Expirations or Exercise		33,216		104,554
Notional Amount	(14,881)	(7,128)	(14,881)	(7,128)
Amount Gain/(Loss) Recognized in Income on Derivatives	67	213	391	337
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Settlement, Expirations or Exercise	29,756		68,320
Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount			(40,525)
Additions	(2,400)	(1,450)	(5,950)	(4,950)
Settlement, Expirations or Exercise	3,375	1,200	12,325	3,475
Notional Amount	(34,150)		(34,150)
Amount Gain/(Loss) Recognized in Income on Derivatives	153	(966)	(1,208)	(1,278)
US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	15	(23)	(106)	(28)
Other derivative instruments [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	1		15
Not Designated as Hedging Instrument [Member] | Receiver Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount				(4,250)
Additions				0
Settlement, Expirations or Exercise				4,250
Notional Amount		0		0
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(1,050)	(5,450)	(2,150)	(6,800)
Notional Amount	(700)	(3,650)	(700)	(3,650)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(35,125)	(44,925)	(40,525)	(43,700)
Notional Amount	(34,150)	(45,175)	(34,150)	(45,175)
Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(2,930)	(2,250)	(1,714)	(5,392)
Additions	2,696	(4,335)	(5,329)	(10,732)
Settlement, Expirations or Exercise	(270)	5,284	1,687	14,823
Notional Amount	(5,356)	(1,301)	(5,356)	(1,301)
Amount Gain/(Loss) Recognized in Income on Derivatives	14	(25)	(142)	(89)
Long [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Trading Securities	45		45
Long [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	1	29	7	(35)
Trading Securities		775		775	$ 62	$ 25	$ 5,192	$ 2,411
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)
Future [Member] | Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(1,960)	(730)	(1,860)	(730)
Additions	(1,960)	(730)	(5,880)	(2,190)
Settlement, Expirations or Exercise	1,960	730	5,780	2,190
Notional Amount	$ (1,960)	$ (730)	$ (1,960)	$ (730)