Derivative Instruments (Open Credit Default Swap Liabilities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013 item		Dec. 31, 2012 item
Open credit default swap liabilities
Credit default swaps, number of instruments	7		7
Fair Value of open credit default swap liabilities	$ (11)	[1]	$ (11)	[1]
Maximum potential payout of open credit default swap liabilities	149		149
BBB- Average Credit Rating [Member] | 12/20/2016 Maturity [Member]
Open credit default swap liabilities
Maturity date of open credit default swap liabilities	Dec. 20, 2016	[2]	Dec. 20, 2016	[2]
Credit rating of underlying obligation	BBB-	[2],[3]	BBB-	[2],[3]
Credit default swaps, number of instruments	3	[2]	3	[2]
Fair Value of open credit default swap liabilities	(4)	[1],[2]	(4)	[1],[2]
Maximum potential payout of open credit default swap liabilities	68	[2]	68	[2]
BBB- Average Credit Rating [Member] | 3/20/2017 Maturity [Member]
Open credit default swap liabilities
Maturity date of open credit default swap liabilities	Mar. 20, 2017	[2]	Mar. 20, 2017	[2]
Credit rating of underlying obligation	BBB-	[2],[3]	BBB-	[2],[3]
Credit default swaps, number of instruments	4	[2]	4	[2]
Fair Value of open credit default swap liabilities	(7)	[1],[2]	(7)	[1],[2]
Maximum potential payout of open credit default swap liabilities	$ 81	[2]	$ 81	[2]

[1] Broker quotes are used to determine the market value of credit default swaps.

[2] These credit default swaps were sold to a counter-party of the consolidated VIEs discussed in Note 4 in our 2012 Form 10-K.

[3] Represents average credit ratings based on the midpoint of the applicable ratings among Moody's, S&P and Fitch Ratings, as scaled to the corresponding S&P ratings.