Restructuring - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Positions	Dec. 31, 2016 USD ($) Positions
Restructuring Plan, 2017
Restructuring Cost and Reserve [Line Items]
Workforce reduction, positions | Positions	520
Initial Charge Recorded	$ 44
Restructuring charges paid	5
Restructuring Plan, 2016
Restructuring Cost and Reserve [Line Items]
Workforce reduction, positions | Positions		230
Restructuring reserve reversed	7
Initial Charge Recorded	30	$ 30
Restructuring charges paid	$ 15	7
Restructuring Plan, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring reserve reversed		7
Initial Charge Recorded		$ 63