Segment Information - Summary of Enterprise-wide Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Revenue by Geographic Region:
Operating revenue	$ 3,469	$ 3,479	$ 3,609	$ 3,552	$ 3,580	$ 3,613	$ 3,831	$ 3,744	$ 14,109	$ 14,768	$ 14,314
United States
Operating Revenue by Geographic Region:
Operating revenue									6,507	6,562	6,243
Canada/Mexico
Operating Revenue by Geographic Region:
Operating revenue									972	1,050	996
Total North America
Operating Revenue by Geographic Region:
Operating revenue									7,479	7,612	7,239
Europe, Middle East and Africa
Operating Revenue by Geographic Region:
Operating revenue									3,920	4,241	4,102
Asia Pacific
Operating Revenue by Geographic Region:
Operating revenue									2,400	2,573	2,577
South America
Operating Revenue by Geographic Region:
Operating revenue									$ 310	$ 342	$ 396