Derivative Instruments (Open Credit Default Swap Liabilities) (Details) - Open Credit Default Swap Liabilities [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item
Summary Of Credit Derivatives
Number of instruments | item	1
Fair value	$ 1	[1]
Maximum potential payout	$ 52
3/20/2017 Maturity [Member]
Summary Of Credit Derivatives
Credit rating of underlying obligation	BBB+	[2]
Number of instruments | item	1
Fair value	$ 1	[1]
Maximum potential payout	$ 52

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