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Revenues - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Contract payment period     15 days
Contract liabilities $ 0   $ 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) 4.00%   5.00%