Unaudited Quarterly Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues	$ 697	$ 827	$ 705	$ 617	$ 679	$ 839	$ 698	$ 630	$ 2,846	$ 2,846	$ 2,577
Operating income	105	244	158	101	113	232	145	106	608	596	614
Net income	$ 37	$ 124	$ 69	$ 31	$ 41	$ 119	$ 57	$ 35	$ 261	$ 252	$ 269