Segment Information - Operating Revenue by Geographic Region (Details) - USD ($) $ 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
Operating Revenue by Geographic Region:
Operating revenue	$ 3,580	$ 3,613	$ 3,831	$ 3,744	$ 3,629	$ 3,615	$ 3,599	$ 3,471	$ 14,768	$ 14,314	$ 13,599
United States
Operating Revenue by Geographic Region:
Operating revenue									6,562	6,243	6,176
Canada/Mexico
Operating Revenue by Geographic Region:
Operating revenue									1,050	996	923
Total North America
Operating Revenue by Geographic Region:
Operating revenue									7,612	7,239	7,099
Europe, Middle East and Africa
Operating Revenue by Geographic Region:
Operating revenue									4,241	4,102	3,787
Asia Pacific
Operating Revenue by Geographic Region:
Operating revenue									2,573	2,577	2,361
South America
Operating Revenue by Geographic Region:
Operating revenue									$ 342	$ 396	$ 352