Restructuring - Changes in Restructuring Liability (Footnote) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Impairment of right-of-use		$ 21
Operating lease right-of-use assets		393	$ 456
Restructuring liability		21	$ 24	$ 42
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201802Member
Cumulative Effect, Period of Adoption, Adjustment
Restructuring Cost and Reserve [Line Items]
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201802Member	us-gaap:AccountingStandardsUpdate201602Member
Cumulative Effect, Period of Adoption, Adjustment | 2018 Restructuring Program
Restructuring Cost and Reserve [Line Items]
Restructuring liability				$ (11)
Real Estate
Restructuring Cost and Reserve [Line Items]
Impairment of right-of-use		21	$ 25
Real Estate | Level 3
Restructuring Cost and Reserve [Line Items]
Operating lease right-of-use assets		10	18
Contract Termination Costs
Restructuring Cost and Reserve [Line Items]
Restructuring liability		3	3	12
Contract Termination Costs | Cumulative Effect, Period of Adoption, Adjustment | 2018 Restructuring Program
Restructuring Cost and Reserve [Line Items]
Restructuring liability				(11)
Contract Termination Costs | Real Estate
Restructuring Cost and Reserve [Line Items]
Non-cash acceleration of amortization		$ 13	$ 4
Contract Termination Costs | Real Estate | Cumulative Effect, Period of Adoption, Adjustment | 2018 Restructuring Program
Restructuring Cost and Reserve [Line Items]
Restructuring liability				$ (11)