Segment Information - Operating Revenue by Geographic Region (Details) - USD ($) $ 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
Operating Revenue by Geographic Region:
Operating revenue	$ 3,629	$ 3,615	$ 3,599	$ 3,471	$ 3,399	$ 3,495	$ 3,431	$ 3,274	$ 14,314	$ 13,599	$ 13,405
United States
Operating Revenue by Geographic Region:
Operating revenue									6,243	6,176	6,167
Canada/Mexico
Operating Revenue by Geographic Region:
Operating revenue									996	923	928
Total North America
Operating Revenue by Geographic Region:
Operating revenue									7,239	7,099	7,095
Europe, Middle East and Africa
Operating Revenue by Geographic Region:
Operating revenue									4,102	3,787	3,725
Asia Pacific
Operating Revenue by Geographic Region:
Operating revenue									2,577	2,361	2,197
South America
Operating Revenue by Geographic Region:
Operating revenue									$ 396	$ 352	$ 388