Quarterly Financial Data (Unaudited) - Summarized Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 1,385	$ 1,375	$ 1,343	$ 1,254	$ 1,234	$ 1,209	$ 1,132	$ 1,001	$ 5,357	$ 4,576	$ 4,146
Income from operations	516	477	503	421	505	479	415	336	$ 1,917	$ 1,735	$ 1,667
Net income	$ 301	$ 250	$ 314	$ 279	$ 578	$ 247	$ 243	$ 167
Basic earnings (loss) per share (in dollars per share)	$ 0.65	$ 0.53	$ 0.67	$ 0.60	$ 1.64	$ 0.39	$ 0.38	$ 0.21	$ 2.46	$ 2.64	$ 1.48
Diluted earnings (loss) per share (in dollars per share)	$ 0.64	$ 0.53	$ 0.66	$ 0.59	$ 1.59	$ 0.37	$ 0.37	$ 0.21	$ 2.42	$ 2.54	$ 1.45