Restructuring Restructuring - Restructuring Expense by Type (Details) - 2016 Restructuring [Member] - USD ($) $ in Millions	12 Months Ended	17 Months Ended
	Dec. 31, 2017	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Severance benefits	$ 34	$ 60
Asset write-off costs	16	16
Transition costs	17	17
Other costs	15	23
Total restructuring expense	$ 82	$ 116