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Business Segments, Concentration of Credit Risk and Significant Customers - Additional Information (Details)
6 Months Ended 12 Months Ended
Jun. 30, 2018
Segment
Customer
Dec. 31, 2017
Customer
Business Segments    
Number of business segments | Segment 1  
Number of measurements of profitability 1  
Net accounts receivable | One Customer | Credit concentration    
Business Segments    
Percentage of concentration risk 76.00% 63.00%
Number of customers | Customer 2 1