Unaudited Quarterly Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues	$ 731	$ 909	$ 763	$ 656	$ 697	$ 827	$ 705	$ 617	$ 3,059	$ 2,846	$ 2,846
Operating income	132	258	158	79	105	244	158	101	627	608	596
Net income	$ 52	$ 136	$ 71	$ 17	$ 37	$ 124	$ 69	$ 31	$ 276	$ 261	$ 252