Annual report pursuant to Section 13 and 15(d)

Debt and Capital Lease Obligations (Details)

v3.8.0.1
Debt and Capital Lease Obligations (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Sep. 29, 2017
Feb. 04, 2015
Line of Credit Facility [Line Items]          
Credit facility term (in years)       5  
Senior credit facility amount       $ 250,000 $ 275,000
Additional borrowing capacity of credit facility       $ 100,000  
Base reference rate of credit facilities LIBOR        
Debt Instrument, Basis Spread on Variable Rate 1.00%        
Utilized for outstanding letters of credit $ 7,932        
Available borrowing capacity 101,568        
Proceeds from Issuance of Long-term Debt 0 $ 0 $ 125,000    
Revolving Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Senior credit facility amount         150,000
Basis spread on variable rate minimum (in hundredths)       0.30%  
Basis spread on variable rate maximum (in hundredths)       0.80%  
Proceeds from Lines of Credit 40,500        
Available borrowing capacity $ 150,000        
Line of Credit Facility, Interest Rate at Period End 2.90%        
Term loan [Member]          
Line of Credit Facility [Line Items]          
Senior credit facility amount         $ 125,000
Letter of Credit [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases       $ 30,000  
Swing line loan [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases       $ 30,000  
Federal funds [Member]          
Line of Credit Facility [Line Items]          
Debt Instrument, Basis Spread on Variable Rate 0.50%