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Debt (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Apr. 27, 2018
Apr. 26, 2018
Oct. 29, 2015
May 09, 2013
Line of Credit Facility [Line Items]                
Unsecured revolving line of credit               $ 50,000,000.0
Ratio of consolidated indebtedness to earnings before interest, taxes, depreciation and amortization, maximum allowed     3.25          
Ratio of consolidated earnings before interest, taxes, depreciation and amortization expense, minimum allowed     3          
Interest rate in event of default     2.00%          
Outstanding borrowing on line of credit $ 0   $ 0          
Interest expense $ 0 $ 179,000 $ 0 $ 343,000        
Weighted average interest rate 3.20% 2.30% 3.20% 2.30%        
First Amendment                
Line of Credit Facility [Line Items]                
Unsecured revolving line of credit             $ 125,000,000.0  
Optional credit line increase             $ 25,000,000.0  
Second Amendment                
Line of Credit Facility [Line Items]                
Unsecured revolving line of credit         $ 5,000,000.0 $ 125,000,000.0    
Letter of credit sublimit         $ 5,000,000.0 $ 10,000,000.0    
Minimum                
Line of Credit Facility [Line Items]                
Quarterly commitment fee     0.175%          
Maximum                
Line of Credit Facility [Line Items]                
Quarterly commitment fee     0.30%          
Base Rate | Minimum                
Line of Credit Facility [Line Items]                
Variable interest rate spread     0.00%          
Base Rate | Maximum                
Line of Credit Facility [Line Items]                
Variable interest rate spread     0.50%          
London Interbank Offered Rate | Minimum                
Line of Credit Facility [Line Items]                
Variable interest rate spread     1.125%          
London Interbank Offered Rate | Maximum                
Line of Credit Facility [Line Items]                
Variable interest rate spread     2.00%