Derivative Instruments (Open Credit Default Swap Liabilities) (Details) - Open Credit Default Swap Liabilities [Member] $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Summary Of Credit Derivatives
Number of instruments | item		5	5
Fair value	[1]	$ (12)	$ (9)
Maximum potential payout		$ 103	$ 103
BBB- average credit rating | 12/20/2016 maturity
Summary Of Credit Derivatives
Credit rating of underlying obligation	[2]	BB+	BBB-
Number of instruments | item		2	2
Fair value	[1]	$ (1)	$ (2)
Maximum potential payout		$ 45	$ 45
BBB- average credit rating | 3/20/2017 maturity
Summary Of Credit Derivatives
Credit rating of underlying obligation	[2]	BB	BBB-
Number of instruments | item		3	3
Fair value	[1]	$ (11)	$ (7)
Maximum potential payout		$ 58	$ 58

[1] Broker quotes are used to determine the market value of these 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.