Revenue - Narrative (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
Revenue, Contract Assets, Liabilities and Costs[Line Items]
Contract liabilities	$ 134		$ 134		$ 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	$ 165		$ 165		172
Contract Fulfillment Costs and Inducements
Revenue, Contract Assets, Liabilities and Costs[Line Items]
Capitalized contract costs, amounts deferred	12		12		$ 12
Capitalized contract costs, amortization	$ 2	$ 2	$ 3	$ 3