Restructuring Restructuring - Restructuring Expense by Type (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense		$ 55	$ 4
Cumulative Amounts Incurred to Date	$ 260
2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	8	30	82
Cumulative Amounts Incurred to Date	154
Severance Benefits | Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	39	15	4
Cumulative Amounts Incurred to Date	58
Severance Benefits | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	0	9	34	$ 26
Cumulative Amounts Incurred to Date	69
Asset write-off costs | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	0	1	16
Cumulative Amounts Incurred to Date	17
Transition Costs | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	0	7	17
Cumulative Amounts Incurred to Date	24
Other costs | Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	162	40	0
Cumulative Amounts Incurred to Date	202
Other costs | 2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	8	13	$ 15	$ 8
Cumulative Amounts Incurred to Date	44
Continuing Operations | Organizational restructuring
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	$ 201	$ 49