Annual report pursuant to Section 13 and 15(d)

Line of Credit (Details)

v3.8.0.1
Line of Credit (Details)
1 Months Ended 12 Months Ended
Jun. 09, 2017
USD ($)
Jan. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Mar. 09, 2018
USD ($)
Feb. 26, 2018
USD ($)
Dec. 28, 2017
USD ($)
Jun. 08, 2017
USD ($)
Line of Credit Facility [Line Items]                  
Revolving credit facility $ 40,000,000                
Financial covenants, minimum net worth     $ 200,000,000.0         $ 230,000,000  
Financial covenants, minimum current ratio     1.25            
Financial covenants, percent of net income added to net worth requirement     50.00%            
Financial covenants, percent of proceeds from stock issuance added to net worth requirement     100.00%            
Financial covenants, maximum funded debt to tangible net worth ratio     0.5            
Outstanding letters of credit $ 5,500,000               $ 4,600,000
Borrowings under credit agreement     $ 0            
Borrowings against credit agreement     2,000,000 $ 0 $ 0        
Revolving Credit Facility                  
Line of Credit Facility [Line Items]                  
Fees on unused borrowings 0.40%                
Revolving Credit Facility | London Interbank Offered Rate (LIBOR)                  
Line of Credit Facility [Line Items]                  
Basis spread on variable interest rate 2.00%                
Letter of Credit                  
Line of Credit Facility [Line Items]                  
Fees on unused borrowings 2.00%                
Revolving credit facility, unused portion     $ 34,500,000            
Subsequent Event                  
Line of Credit Facility [Line Items]                  
Financial covenants, minimum net worth             $ 185,000,000.0    
Percent of gain on sale of assets added to net worth requirement             50.00%    
Borrowings under credit agreement           $ 10,000,000.0      
Borrowings against credit agreement   $ 10,000,000