Unaudited Quarterly Information (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
Revenues	$ 765		$ 843		$ 752		$ 638		$ 731	$ 909	$ 763	$ 656	$ 2,998	$ 3,059	$ 2,846
Operating income	123		247		164		77		132	258	158	79	611	627	608
Net income	$ 210	[1]	$ 130	[1]	$ 75	[1]	$ 18	[1]	$ 52	$ 136	$ 71	$ 17	433	276	261
Federal income tax rate reduction due to tax reform													$ 158	$ 0	$ 0

[1] Net income for the fourth quarter 2017 includes a reduction in income taxes of $158 million due to tax reform. See Note 9 for further discussion of the impacts of tax reform implementation.