Revenues - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract payment period	15 days
Contract liabilities	$ 0
Impairment loss	$ 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]
Revenue from other services (as a percent)	5.00%