Derivative Instruments (Open Credit Default Swap Liabilities) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) item	Dec. 31, 2018 security
Summary Of Credit Derivatives
Number of instruments | security			0
Open Credit Default Swap Liabilities [Member]
Summary Of Credit Derivatives
Maximum potential payout		$ 52
BBB+ [Member] | Maturity 12/20/2022 [Member] | Open Credit Default Swap Liabilities [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.