Derivative Instruments (Open Credit Default Swap Liabilities) (Details) - Open Credit Default Swap Liabilities [Member] $ in Millions		12 Months Ended
		Dec. 31, 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		$ 52	$ 40
BBB+ [Member] | 3/20/2017 Maturity [Member]
Summary Of Credit Derivatives
Credit rating of underlying obligation			BBB+
Number of instruments | item			2
Maximum potential payout			$ 40
BBB+ [Member] | Maturity 12/20/2022 [Member]
Summary Of Credit Derivatives
Credit rating of underlying obligation		BBB+
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.