Supplementary Data - Quarterly Financial Data - Unaudited (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,706	$ 3,578	$ 3,689	$ 3,513	$ 3,559	$ 3,460	$ 3,484	$ 3,293	$ 14,486	$ 13,796	$ 12,784
Operating income	310	339	369	332	291	298	316	254	1,350	1,159	988
Net income	$ 202	$ 210	$ 240	$ 207	$ 197	$ 192	$ 192	$ 172	$ 859	$ 753	$ 626
Diluted earnings per share (in USD per share)	$ 0.77	$ 0.78	$ 0.87	$ 0.73	$ 0.68	$ 0.65	$ 0.64	$ 0.57	$ 3.15	$ 2.54	$ 2.00