Restructuring Restructuring - Accrued Liability for Restructuring Expense (Details) - 2016 Restructuring [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Restructuring Reserve [Roll Forward]
Accrued liability as of January 1, 2017	$ 23
Provision	71
Payments	(39)
Other adjustments(1)	(5)
Accrued liability as of December 31, 2017	50
Severance Benefits
Restructuring Reserve [Roll Forward]
Accrued liability as of January 1, 2017	21
Provision	39
Payments	(25)
Other adjustments(1)	(5)
Accrued liability as of December 31, 2017	30
Transition Costs
Restructuring Reserve [Roll Forward]
Accrued liability as of January 1, 2017	0
Provision	17
Payments	0
Other adjustments(1)	0
Accrued liability as of December 31, 2017	17
Other Costs
Restructuring Reserve [Roll Forward]
Accrued liability as of January 1, 2017	2
Provision	15
Payments	(14)
Other adjustments(1)	0
Accrued liability as of December 31, 2017	$ 3