Restructuring - Restructuring Expense by Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses		$ 13		$ 31
Cumulative Amounts Incurred to Date	$ 223		$ 223
2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	2	10	8	27
Cumulative Amounts Incurred to Date	154		154
Severance benefits | Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	(9)	0	42	5
Cumulative Amounts Incurred to Date	61		61
Severance benefits | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	1	5	1	9
Cumulative Amounts Incurred to Date	70		70
Asset write-off costs | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	0	0	0	0
Cumulative Amounts Incurred to Date	17		17
Transition Costs | Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses			122
Transition Costs | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	(1)	1	0	7
Cumulative Amounts Incurred to Date	24		24
Other costs | Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	35	13		26
Cumulative Amounts Incurred to Date	162		162
Other costs | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expenses	2	$ 4	7	$ 11
Cumulative Amounts Incurred to Date	$ 43		$ 43