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Revolving Credit Agreement (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Assets $ 931,680   $ 814,514
Revolving Credit Facility      
Debt Instrument [Line Items]      
Line of credit facility, current borrowing capacity 530,800 $ 400,000  
Line of credit facility, maximum borrowing capacity 600,000    
Line of credit facility, additional borrowing capacity $ 650,000    
Line of credit facility, expiration date May 31, 2020    
Line of credit, amount outstanding $ 370,000    
Letters of credit outstanding 6,900   $ 6,300
Line of credit facility, remaining borrowing capacity $ 153,900    
Base spread on variable rate 3.15%    
Minimum | Guarantor Subsidiaries | Revolving Credit Facility      
Debt Instrument [Line Items]      
Assets $ 500    
LIBOR | Revolving Credit Facility      
Debt Instrument [Line Items]      
Variable interest rate 1.17%