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Debt (Details)
 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
note
Dec. 31, 2016
USD ($)
Line of Credit Facility [Line Items]      
Total outstanding $ 1,800,000 $ 4,721,000  
2017 Insurance Note Payable | Insurance Notes Payable      
Line of Credit Facility [Line Items]      
Number of notes financed | note   3  
Face amount   $ 5,100,000  
Interest rate   2.90%  
Total outstanding 1,800,000 $ 4,700,000  
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit 100,000,000.0    
Maximum additional borrowing capacity 150,000,000.0    
Letters of credit, amount outstanding 2,300,000 2,800,000  
Credit facility, amount outstanding $ 0 0  
Borrowing capacity multiplier 2.5    
Maximum debt to adjusted EBITDA ratio 2.50    
Minimum EBITDA to interest expense ratio 3.0    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Minimum      
Line of Credit Facility [Line Items]      
Unused capacity, commitment fee 0.25%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Maximum      
Line of Credit Facility [Line Items]      
Unused capacity, commitment fee 0.375%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 1, Base Rate | Federal Funds Effective Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 0.50%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 1, Base Rate | Eurodollar      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.00%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 1, Base Rate | Eurodollar | Minimum      
Line of Credit Facility [Line Items]      
Additional spread on variable rate 0.50%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 1, Base Rate | Eurodollar | Maximum      
Line of Credit Facility [Line Items]      
Additional spread on variable rate 1.50%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 2, Adjusted Eurodollar Rate | Eurodollar      
Line of Credit Facility [Line Items]      
Threshold period for change in interest payment schedule 3 months    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 2, Adjusted Eurodollar Rate | Eurodollar | Minimum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.50%    
Revolving Credit Facility | Credit Facility | Revolving Credit Facility Maturing August 2018 | Interest Option 2, Adjusted Eurodollar Rate | Eurodollar | Maximum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 2.50%    
Letter of Credit | Credit Facility | Revolving Credit Facility Maturing August 2018      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit $ 20,000,000.0    
Swing Line Loan | Credit Facility | Revolving Credit Facility Maturing August 2018      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit 10,000,000.0    
Citibank Credit Facility | Credit Facility      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit     $ 6,000,000.0
Letters of credit, amount outstanding $ 4,900,000 $ 2,600,000  
Expiration period     3 years
Fixed interest rate, short-term     1.25%
Fixed interest rate, long-term     1.50%