Revenues - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract payment period	15 days
Remaining performance obligations	$ 0
Contract assets	0
Contract liability	0
Deferred contract costs	0
Impairment losses	$ 0	$ 0
Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transit time	3 days
Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transit time	8 days
Customer Concentration Risk | Sales Revenue
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Percentage of revenue from other services	6.00%