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Debt (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
note
Dec. 31, 2016
USD ($)
Sep. 30, 2018
USD ($)
Line of Credit Facility [Line Items]      
Total outstanding $ 4,721,000   $ 432,000
2017 Insurance Note Payable | Insurance Notes Payable      
Line of Credit Facility [Line Items]      
Number of notes financed | note 3    
Face amount $ 5,100,000.0    
Interest rate 2.90%    
Total outstanding $ 4,700,000   400,000
Revolving Credit Facility | Line of Credit | Revolving Credit Facility Maturing August 2018      
Line of Credit Facility [Line Items]      
Letters of credit, amount outstanding 2,800,000   2,800,000
Maximum borrowing capacity limit     100,000,000.0
Letter of Credit | Line of Credit | Revolving Credit Facility Maturing August 2018      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit     20,000,000.0
Swing Line Loan | Line of Credit | Revolving Credit Facility Maturing August 2018      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit     10,000,000.0
Citibank Credit Facility | Line of Credit      
Line of Credit Facility [Line Items]      
Letters of credit, amount outstanding $ 2,600,000   $ 2,300,000
Maximum borrowing capacity limit   $ 6,000,000.0  
Expiration period   3 years  
Fixed interest rate, short-term   1.25%  
Fixed interest rate, long-term   1.50%