Derivative Instruments (Open Credit Default Swap Liabilities) (Details) - Open Credit Default Swap Liabilities [Member] $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Summary Of Credit Derivatives
Number of instruments | item		1	2
Fair value	[1]	$ 1
Maximum potential payout		$ 63	$ 40
3/20/2017 maturity
Summary Of Credit Derivatives
Credit rating of underlying obligation	[2]		BBB+
Number of instruments | item			2
Maximum potential payout			$ 40
6/20/2022 maturity
Summary Of Credit Derivatives
Credit rating of underlying obligation	[2]	BBB+
Number of instruments | item		1
Fair value	[1]	$ 1
Maximum potential payout		$ 63

[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.