Quarterly Supplementary Data (Details) - USD ($) $ / shares in Units, $ 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	$ 1,411	$ 1,412	$ 1,386	$ 1,523	$ 1,312	$ 1,402	$ 1,371	$ 1,364	$ 5,732	$ 5,449	$ 5,142
Income before income taxes	197	129	128	276	128	253	166	265	730	812	881
Net Income (Loss) Attributable to Parent	$ 642	$ 102	$ 100	$ 201	$ 100	$ 180	$ 123	$ 188	$ 1,045	$ 591	$ 634
Basic (in usd per share)	$ 3.92	$ 0.62	$ 0.61	$ 1.22	$ 0.61	$ 1.09	$ 0.75	$ 1.14	$ 6.36	$ 3.59	$ 3.87
Diluted (in usd per share)	$ 3.88	$ 0.61	$ 0.60	$ 1.21	$ 0.60	$ 1.08	$ 0.74	$ 1.13	$ 6.29	$ 3.55	$ 3.83