Restructuring Costs - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended				45 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2017 USD ($) Position
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 0	$ 31	$ 0	$ 85
Operational Improvement Program [Member]
Restructuring Cost and Reserve [Line Items]
Number of support roles moved from higher cost locations to lower cost locations (more than) | Position									3,500
Restructuring costs		$ 31		$ 85	$ 134	$ 145	$ 126	$ 36
Cumulative restructuring cost									$ 441