Quarterly Results (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 848	$ 804	$ 816	$ 751	$ 751	$ 713	$ 742	$ 685	$ 3,219	$ 2,891	$ 3,422
Net income	$ 650	$ 140	$ 120	$ 86	$ 108	$ 65	$ 94	$ 52	996	$ 319	$ 561
Tax Cuts And Jobs Act Of 2017, Deferred tax asset, provisional income tax benefit									$ 514