Restructuring - Changes in Restructuring Liability (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended				26 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	$ 42	$ 24	$ 42
Restructuring liability, Ending Balance		21	$ 24	$ 42		$ 21
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201802Member
Cumulative Effect, Period of Adoption, Adjustment
Restructuring Reserve [Roll Forward]
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201802Member	us-gaap:AccountingStandardsUpdate201602Member
Employee Termination Costs
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	$ 30	21	$ 30
Restructuring liability, Ending Balance		18	21	$ 30		18
Contract Termination Costs
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	12	3	12
Restructuring liability, Ending Balance		3	3	12		3
2018 Restructuring Program
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		(4)	31
Cash payments and adjustments		(18)	(38)
2018 Restructuring Program | Cumulative Effect, Period of Adoption, Adjustment
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	(11)		(11)
Restructuring liability, Ending Balance				(11)
2018 Restructuring Program | Employee Termination Costs
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		(4)	26
Cash payments and adjustments		(17)	(35)			(55)
Cumulative expense incurred to date		55				55
2018 Restructuring Program | Employee Termination Costs | Cumulative Effect, Period of Adoption, Adjustment
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	0		0
Restructuring liability, Ending Balance				0
2018 Restructuring Program | Contract Termination Costs
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		0	5
Cash payments and adjustments		(1)	(3)
Cumulative expense incurred to date		50				50
2018 Restructuring Program | Contract Termination Costs | Cumulative Effect, Period of Adoption, Adjustment
Restructuring Reserve [Roll Forward]
Restructuring liability, Beginning Balance	$ (11)		$ (11)
Restructuring liability, Ending Balance				$ (11)
2020 Real Estate Rationalization Restructuring Program
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		1
2020 Real Estate Rationalization Restructuring Program | Employee Termination Costs
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		0
Cumulative expense incurred to date		0				0
2020 Real Estate Rationalization Restructuring Program | Contract Termination Costs
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		1
Cumulative expense incurred to date		36				36
2020 MA Strategic Reorganization Restructuring Program
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		18
2020 MA Strategic Reorganization Restructuring Program | Employee Termination Costs
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		18
Cumulative expense incurred to date		18				18
2020 MA Strategic Reorganization Restructuring Program | Contract Termination Costs
Restructuring Reserve [Roll Forward]
Cost incurred and adjustments		0
Cumulative expense incurred to date		$ 0				$ 0