Basis of Presentation - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Contract Acquisition Costs
Deferred contract costs	$ 160		$ 160		$ 153
Right-of-use assets obtained in exchange for operating lease obligations			$ 80	$ 146
Minimum [Member]
Contract Acquisition Costs
Deferred revenue recognition period			1 month
Contract amortization period	5 years		5 years
Maximum [Member]
Contract Acquisition Costs
Deferred revenue recognition period			3 months
Contract amortization period	13 years		13 years
Selling, General and Administrative Expenses [Member]
Contract Acquisition Costs
Deferred contract costs amortization	$ 6	$ 6	$ 17	$ 17
Deferred Sales Incentives [Member]
Contract Acquisition Costs
Deferred contract costs	$ 117		$ 117		$ 117