Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Revenue, Contract Assets, Liabilities and Costs[Line Items]
Contract liabilities	$ 138		$ 138		$ 116
Contract liability amortization period			30 months
Deferred incremental direct costs, amortization period	4 years		4 years
Deferred incremental direct costs net of accumulated amortization	$ 162		$ 162		172
Contract Fulfillment Costs and Inducements
Revenue, Contract Assets, Liabilities and Costs[Line Items]
Capitalized contract costs, amounts deferred	13		13		$ 12
Capitalized contract costs, amortization	$ 1	$ 0	$ 4	$ 3