Derivative and Other Hedging Instruments (Effect Of Derivative Instruments Not Designated As Hedges On Comprehensive Income Statement) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	$ (181)	$ 191	$ (323)
Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(6,650)
Additions	(1,250)	(6,450)	(500)
Settlement, Expirations or Exercise	4,400	1,000	1,450
Notional Amount	(3,500)	(6,650)
Amount Gain/(Loss) Recognized in Income on Derivatives	90	(66)	(3)
TBA and Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(15,474)	(10,916)	(7,295)
Additions	(194,534)	(237,601)	(116,439)
Settlement, Expirations or Exercise		233,043	112,818
Notional Amount	(7,152)	(15,474)	(10,916)
Amount Gain/(Loss) Recognized in Income on Derivatives	(299)	330	(59)
Derivative, Notional Amount	(7,152)	(15,474)
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Settlement, Expirations or Exercise	202,856
Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(43,700)
Additions	(14,350)	(14,825)	(15,650)
Settlement, Expirations or Exercise	6,425	8,300	19,000
Notional Amount	(51,625)	(43,700)
Amount Gain/(Loss) Recognized in Income on Derivatives	140	67	(397)
US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	48	0	(5)
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(3,500)
Derivative, Notional Amount		(6,650)	(1,200)	$ (2,150)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(51,625)
Derivative, Notional Amount		(43,700)	(37,175)	(40,525)
Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(10,699)	(8,061)	(1,714)
Additions	19,278	(14,030)	(9,884)
Settlement, Expirations or Exercise	(8,632)	11,392	3,537
Notional Amount	(21,345)	(10,699)	(8,061)
Amount Gain/(Loss) Recognized in Income on Derivatives	(161)	(141)	7
Derivative, Notional Amount	(1,650)	(2,910)
Long [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Debt Securities, Trading, and Equity Securities, FV-NI	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	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)
Future [Member] | Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(2,910)	(1,810)	(1,860)
Additions	7,859	(11,340)	(7,840)
Settlement, Expirations or Exercise	(9,119)	10,240	7,890
Notional Amount	$ (1,650)	$ (2,910)	$ (1,810)