Segments and Geographic Information - Narrative (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Number of operating segments | segment									2
Operating revenue	$ (10,245)	$ (11,060)	$ (10,791)	$ (9,148)	$ (9,458)	$ (10,483)	$ (10,447)	$ (9,251)	$ (41,244)	$ (39,639)	$ (40,704)
Intersegment Sales/Other | Exchanged products
Business Acquisition [Line Items]
Operating revenue									$ 3,240	$ 2,658	$ 3,108