Restructuring - Changes in Restructuring Liability (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	$ 42	$ 0
Restructuring liability, Ending Balance	24	42
Employee Termination Costs
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	30	0
Restructuring liability, Ending Balance	21	30
Contract Termination Costs
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	12	0
Restructuring liability, Ending Balance	3	12
2018 Restructuring Program
Restructuring Reserve [Roll Forward]
Adoption of New Lease Accounting Standard	(11)
Cost incurred and adjustments	31	45
Cash payments and adjustments	(38)	(3)
2018 Restructuring Program | Employee Termination Costs
Restructuring Reserve [Roll Forward]
Adoption of New Lease Accounting Standard	0
Cost incurred and adjustments	26	33
Cash payments and adjustments	(35)	(3)
Cumulative expense incurred to date	59
2018 Restructuring Program | Contract Termination Costs
Restructuring Reserve [Roll Forward]
Adoption of New Lease Accounting Standard	(11)
Cost incurred and adjustments	5	12
Cash payments and adjustments	(3)	$ 0
Cumulative expense incurred to date	$ 50