Restructuring - Changes in Restructuring Liability (Footnote) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Jan. 01, 2019
Restructuring Cost and Reserve [Line Items]
Impairment of right-of-use	$ 25
Operating lease right-of-use assets			$ 456
Accounting Standards Update 2016-02
Restructuring Cost and Reserve [Line Items]
Operating lease right-of-use assets				$ 518
Real Estate
Restructuring Cost and Reserve [Line Items]
Impairment of right-of-use			25
Real Estate | Level 3
Restructuring Cost and Reserve [Line Items]
Operating lease right-of-use assets			18
Contract Termination Costs | Real Estate
Restructuring Cost and Reserve [Line Items]
Non-cash acceleration of amortization			$ 4
Contract Termination Costs | Real Estate | Accounting Standards Update 2016-02
Restructuring Cost and Reserve [Line Items]
Reclassification of liabilities as a reduction of the ROU Asset capitalized upon adoption		$ 11