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Debt (Details) - Credit Facility
 6 Months Ended 12 Months Ended
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Apr. 28, 2017
USD ($)
Citibank Credit Facility      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit   $ 6,000,000.0  
Letters of credit, amount outstanding $ 3,000,000 $ 2,200,000  
Line of credit, expiration period (in years)   3 years  
Fixed interest rate, short-term   1.25%  
Fixed interest rate, long-term   1.50%  
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility      
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 4,400,000 $ 3,700,000  
Revolving credit facility, amount outstanding $ 0 $ 0  
Maximum debt to adjusted EBITDA ratio 2.50    
Minimum EBITDA to interest expense ratio 3.0    
Covenant term, minimum liquidity (no less than)     $ 250,000,000
Wavier fee (as a percent)     0.05%
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Minimum      
Line of Credit Facility [Line Items]      
Commitment fee (as a percent) 0.25%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Maximum      
Line of Credit Facility [Line Items]      
Commitment fee (as a percent) 0.375%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Federal Funds Effective Rate | Interest Option 1, Base Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 0.50%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Eurodollar | Interest Option 1, Base Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 1.00%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Eurodollar | Interest Option 1, Base Rate | Minimum      
Line of Credit Facility [Line Items]      
Additional spread on variable rate (as a percent) 0.50%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Eurodollar | Interest Option 1, Base Rate | Maximum      
Line of Credit Facility [Line Items]      
Additional spread on variable rate (as a percent) 1.50%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Eurodollar | Interest Option 2, Adjusted Eurodollar Rate      
Line of Credit Facility [Line Items]      
Threshold for change in interest payment schedule (in months) 3 months    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Eurodollar | Interest Option 2, Adjusted Eurodollar Rate | Minimum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 1.50%    
Revolving Credit Facility Maturing August 2018 | Revolving Credit Facility | Eurodollar | Interest Option 2, Adjusted Eurodollar Rate | Maximum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 2.50%    
Revolving Credit Facility Maturing August 2018 | Letter of Credit      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit $ 20,000,000.0    
Revolving Credit Facility Maturing August 2018 | Swing Line Loan      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity limit $ 10,000,000.0