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Revolving Credit Agreement (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Assets $ 1,170,854 $ 1,079,892
Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Line of credit facility, current borrowing capacity 600,000  
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 $ 510,000  
Letters of credit outstanding 6,000  
Line of credit facility, remaining borrowing capacity $ 84,000  
Base spread on variable rate 3.15%  
Revolving Credit Facility | LIBOR    
Line of Credit Facility [Line Items]    
Variable interest rate 1.80%  
Revolving Credit Facility | Minimum | Guarantor Subsidiaries    
Line of Credit Facility [Line Items]    
Assets $ 500