Basis of Presentation - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Contract Acquisition Costs
Deferred contract costs	$ 149		$ 149		$ 145
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	$ 5	$ 11	$ 11
Reduction to Sales [Member]
Contract Acquisition Costs
Deferred contract costs amortization	5	$ 9	11	$ 19
Deferred Sales Incentives [Member]
Contract Acquisition Costs
Deferred contract costs	$ 113		$ 113		$ 109