Basis of Presentation - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Contract Acquisition Costs
Deferred contract costs	$ 172		$ 172		$ 159
Right-of-use assets obtained in exchange for operating lease obligations			$ 57	$ 80
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
Deferred Sales Incentives [Member]
Contract Acquisition Costs
Deferred contract costs	$ 122		$ 122		$ 118
Deferred contract costs amortization	$ 6	$ 6	$ 17	$ 17