Derivative and Other Hedging Instruments (Summary Of Outstanding Interest Rate Swaps Agreements Not Designated As Hedges) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net		$ 2
Securitites borrowed under reverse purchase agreements, fair value	11,763	899
Notional Amount of Interest Rate Derivatives		0	6,450	2,050
Notional Amount		0	6,450	2,050
Terminations Of Interest Rate Cash Flow Hedge Derivatives		(450)	0
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(1,341)	(462)	44
Hedge De-Designation Of Interest Rate Cash Flow Hedge Derivatives		(23,900)	0
Payer Interest Rate Swaps [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	25,000	6,750	350
Notional Amount Of Derivatives Not Designated As Hedging Instruments additions due to Hedge De-Designations		23,900
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(8,400)	(450)	(300)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	46,850	30,250	50	0
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(1,034)	(119)	(3)
Payer Swaption [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	18,250	5,600	850
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(7,000)	(3,250)	(200)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	14,450	3,200	850	200
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(106)	(63)	19
Treasury Futures [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	3,838
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(4,621)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	0	783
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(90)
Purchases Of TBAs And Forward Settling Agency Securities [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	140,731	51,488	6,662
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(122,576)	(48,300)	(6,747)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	21,855	3,700	512	597
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	444	160	19
Receiver Swaption [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period		250	300
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period		(250)	(400)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments				100
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		(1)	0
Markit IOS Total Return Swaps - Long [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	0	1,195
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(41)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	0	41
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0	(7)
Sale Of TBA And Forward Settling Agency Securities [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	176,905	100,077	16,937
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(171,330)	(97,635)	(16,193)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	9,378	3,803	1,361	617
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(413)	(302)	11
Short Sales Of U S Government Securities [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	36,555	15,794	750
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(25,600)	(15,164)	(500)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	11,835	880	250
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(142)	(133)	(2)
Markit IOS Total Return Swaps - Short [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period	0	685
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period	(206)
Notional Amount of Other Derivatives Not Designated as Hedging Instruments	0	206
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0	14
Derivative liabilities, at fair value [Member]
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(1,264)	(853)
Derivative liabilities, at fair value [Member] | Payer Interest Rate Swaps [Member]
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(1,243)	(795)
Derivative liabilities, at fair value [Member] | Treasury Futures [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	0	(14)
Derivative liabilities, at fair value [Member] | Purchases Of TBAs And Forward Settling Agency Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	(1)	0
Derivative liabilities, at fair value [Member] | Sale Of TBA And Forward Settling Agency Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(20)	(44)
Derivative assets, at fair value [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	301	82
Derivative assets, at fair value [Member] | Payer Interest Rate Swaps [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value		13
Derivative assets, at fair value [Member] | Payer Swaption [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	171	11
Derivative assets, at fair value [Member] | Purchases Of TBAs And Forward Settling Agency Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	116	54
Derivative assets, at fair value [Member] | Markit IOS Total Return Swaps - Long [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	0	1
Derivative assets, at fair value [Member] | Sale Of TBA And Forward Settling Agency Securities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	0	3
Put Option [Member]
Notional Amount Of Derivatives Not Designated As Hedging Instruments Additions During The Period		200	75
Notional Amount Of Derivatives Not Designated As Hedging Instruments Settlement Expiration During The Period		(200)	(75)
Greater Than One Year and Less Than or Equal to Three Years [Member]
Notional Amount of Interest Rate Derivatives	14,600	11,350
Notional Amount	14,600	11,350
Average Fixed Pay Rate	1.23%	1.22%
Average Receive Rate	0.26%	0.30%
Net Estimated Fair Value	(294)	(148)
Interest Rate Derivative Not Designated As Hedging Instruments Average receive Rate	0.26%	0.30%
Derivative, Average Remaining Maturity	2 years	2 years 1 month 6 days
Greater Than Three Years and Less Than or Equal to Five Years [Member]
Notional Amount of Interest Rate Derivatives	20,250	16,700
Notional Amount	20,250	16,700
Average Fixed Pay Rate	1.48%	1.77%
Average Receive Rate	0.29%	0.35%
Net Estimated Fair Value	(666)	(607)
Interest Rate Derivative Not Designated As Hedging Instruments Average receive Rate	0.29%	0.35%
Derivative, Average Remaining Maturity	4 years 1 month 12 days	3 years 10 months 24 days
Greater Than Five Years and Less than or Equal to Seven Years [Member]
Notional Amount of Interest Rate Derivatives	5,600	950
Notional Amount	5,600	950
Average Fixed Pay Rate	1.53%	1.56%
Average Receive Rate	0.34%	0.57%
Net Estimated Fair Value	(163)	(9)
Interest Rate Derivative Not Designated As Hedging Instruments Average receive Rate	0.34%	0.57%
Derivative, Average Remaining Maturity	6 years 1 month 6 days	5 years 8 months 12 days
Greater Than Seven Years and Less than or Equal to Ten Years [Member]
Notional Amount of Interest Rate Derivatives	5,200	1,250
Notional Amount	5,200	1,250
Average Fixed Pay Rate	1.89%	1.99%
Average Receive Rate	0.35%	0.55%
Net Estimated Fair Value	(113)	(18)
Interest Rate Derivative Not Designated As Hedging Instruments Average receive Rate	0.35%	0.55%
Derivative, Average Remaining Maturity	9 years 2 months 10 days	8 years 2 months 12 days
Not Designated as Hedging Instrument [Member]
Notional Amount of Interest Rate Derivatives	46,850	30,250
Notional Amount	46,850	30,250
Average Fixed Pay Rate	1.46%	1.57%
Average Receive Rate	0.29%	0.35%
Net Estimated Fair Value	$ (1,229)	$ (782)
Interest Rate Derivative Not Designated As Hedging Instruments Average receive Rate	0.29%	0.35%
Derivative, Average Remaining Maturity	4 years 4 months 10 days	3 years 6 months