Derivative Instruments (Open Credit Default Swap Liabilities) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013 item		Dec. 31, 2012 item
Summary Of Credit Derivatives
Maximum potential payout of open credit default swap liabilities	$ 126		$ 148
Open Credit Default Swap Liabilities [Member]
Summary Of Credit Derivatives
Credit default swaps, number of instruments	6		7
Fair Value of open credit default swap liabilities	(5)	[1]	(11)	[1]
Maximum potential payout of open credit default swap liabilities	126		148
BBB- Average Credit Rating [Member] | 12/20/2016 Maturity [Member] | Open Credit Default Swap Liabilities [Member]
Summary Of Credit Derivatives
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	(2)	[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 [Member]
Summary Of Credit Derivatives
Credit rating of underlying obligation	BBB-	[2]	BBB-	[2]
Credit default swaps, number of instruments	3		4
Fair Value of open credit default swap liabilities	(3)	[1]	(7)	[1]
Maximum potential payout of open credit default swap liabilities	$ 58		$ 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.