v3.8.0.1
Debt (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
bank
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Line of Credit Facility [Line Items]      
Debt outstanding $ 0 $ 0  
Number of banks to which the Company is party to a credit agreement | bank 2    
Calculated maximum borrowings $ 150,000,000.0    
Available increase in borrowings under credit agreement 100,000,000.0    
Outstanding borrowings 0    
Letters of credit outstanding 1,300,000    
Remaining borrowing capacity 148,700,000    
Total interest paid 200,000 200,000 $ 300,000
Total interest capitalized $ 0 $ 0 $ 0
Minimum | Credit Agreement      
Line of Credit Facility [Line Items]      
Percentage of commitment fee 0.15%    
Maximum | Credit Agreement      
Line of Credit Facility [Line Items]      
Percentage of commitment fee 0.30%    
Credit Agreement Interest Rate Option One | London Interbank Offered Rate (LIBOR) | Minimum | Credit Agreement      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.875%    
Credit Agreement Interest Rate Option One | London Interbank Offered Rate (LIBOR) | Maximum | Credit Agreement      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.75%    
Credit Agreement Interest Rate Option Two | Minimum | Credit Agreement      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on base rate 0.00%    
Credit Agreement Interest Rate Option Two | Maximum | Credit Agreement      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on base rate 0.25%    
Credit Agreement Interest Rate Option Two | London Interbank Offered Rate (LIBOR) | Credit Agreement      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.00%    
Credit Agreement Interest Rate Option Two | Federal Funds Rate | Credit Agreement      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.50%