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, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	$ (3,180)	$ (996)
Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(9,550)	(2,550)	$ (8,850)
Additions	(2,000)	0
Settlement, Expirations or Exercise	1,300	950
Notional Amount	(9,550)
Amount Gain/(Loss) Recognized in Income on Derivatives	(134)	(27)
TBA and Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(20,279)	(6,822)	(7,322)	$ (7,152)
Additions	(37,750)	(18,442)
Settlement, Expirations or Exercise		18,772
Amount Gain/(Loss) Recognized in Income on Derivatives	693	83
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Settlement, Expirations or Exercise	24,793
Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Additions	(49,975)	(5,350)
Settlement, Expirations or Exercise	82,575	8,800
Notional Amount	(46,475)
Amount Gain/(Loss) Recognized in Income on Derivatives	(2,795)	(596)
US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	(104)	(31)
Not Designated as Hedging Instrument [Member] | Interest Rate Swaption [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount				(3,500)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(46,475)	(48,175)	(79,075)	(51,625)
Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(4,245)	(18,735)	(9,224)	(21,345)
Additions	(6,045)	(4,770)
Settlement, Expirations or Exercise	(11,024)	7,380
Amount Gain/(Loss) Recognized in Income on Derivatives	(937)	(425)
Long [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Amount Gain/(Loss) Recognized in Income on Derivatives	97	0
Debt Securities, Trading, and Equity Securities, FV-NI	3,569	120	95	45
Trading Securities Added During the Period	6,461	405
Notional Amount Of Trading Securities Settlement Expiration During The Period	(2,987)	(330)
Future [Member] | Short [Member] | US Treasury Securities [Member]
Changes in Derivative and Other Hedge Portfolio [Roll Forward]
Notional Amount	(1,000)	(1,650)	$ (1,000)	$ (1,650)
Additions	1,000	(1,650)
Settlement, Expirations or Exercise	$ (1,000)	$ 1,650