Derivative Instruments (Open Credit Default Swap Liabilities) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 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	$ (8)	[1]	$ (11)	[1]
Maximum potential payout of open credit default swap liabilities	148		148
BBB- Average Credit Rating [Member] | 12/20/2016 Maturity [Member]
Open credit default swap liabilities
Credit rating of underlying obligation	BBB-	[2]	BBB-	[2]
Credit default swaps, number of instruments	3		3
Fair Value of open credit default swap liabilities	(3)	[1]	(4)	[1]
Maximum potential payout of open credit default swap liabilities	68		68
BBB- Average Credit Rating [Member] | 3/20/2017 Maturity [Member]
Open credit default swap liabilities
Credit rating of underlying obligation	BBB-	[2]	BBB-	[2]
Credit default swaps, number of instruments	4		4
Fair Value of open credit default swap liabilities	(5)	[1]	(7)	[1]
Maximum potential payout of open credit default swap liabilities	$ 80		$ 80

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

[2] 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.