Derivative and Other Hedging Instruments (Summary of Long and Short Position of Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Notional Amount	$ 12,477		$ (103)
Derivative, Forward Settlement Value	12,775		(153)
Fair Value of Derivative Instruments	95		13
TBA securities Fifteen Year and Thirty Year Securities [Member]
Notional Amount	12,327	[1]	(615)	[1]
Derivative, Forward Settlement Value	12,612		(683)
Fair Value of Derivative Instruments	96	[1]	8	[1]
TBA securities Fifteen Year and Thirty Year Securities [Member] | Purchase [Member]
Notional Amount	21,705		3,188
Derivative, Forward Settlement Value	22,603		3,252
Fair Value of Derivative Instruments	116		49
TBA securities Fifteen Year and Thirty Year Securities [Member] | Sale [Member]
Notional Amount	(9,378)		(3,803)
Derivative, Forward Settlement Value	(9,991)		(3,935)
Fair Value of Derivative Instruments	(20)		(41)
Forward settling securities Twenty Year Securities and ARMs [Member]
Notional Amount	150	[2]	512	[2]
Derivative, Forward Settlement Value	163		530
Fair Value of Derivative Instruments	(1)	[2]	5	[2]
Forward settling securities Twenty Year Securities and ARMs [Member] | Purchase [Member]
Notional Amount	150		512
Derivative, Forward Settlement Value	163		530
Fair Value of Derivative Instruments	$ (1)		$ 5

[1] Includes 15-year and 30-year TBA securities of varying coupons

[2] Includes 20-year and 30-year fixed securities of varying coupons