Quarterly report pursuant to Section 13 or 15(d)

Debt and Credit Arrangements - Narrative (Details)

v3.10.0.1
Debt and Credit Arrangements - Narrative (Details)
9 Months Ended
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Line of Credit Facility [Line Items]      
Revolving credit facility $ 84,426,000 $ 154,283,000  
Revolving credit facility | Line of Credit      
Line of Credit Facility [Line Items]      
Revolving credit facility $ 115,000,000 $ 155,000,000  
Credit Agreement | Citibank, National Association      
Line of Credit Facility [Line Items]      
Aggregate secured revolving credit facility     $ 225,000,000.0
Leverage Ratio 1.99    
Interest Coverage Ratio 5.22    
All-in rate 4.38%    
Credit Agreement | Citibank, National Association | Federal Funds Rate      
Line of Credit Facility [Line Items]      
Interest rate 0.50%    
Credit Agreement | Citibank, National Association | LIBOR rate      
Line of Credit Facility [Line Items]      
Interest rate 1.00%    
Credit Agreement | Citibank, National Association | Maximum      
Line of Credit Facility [Line Items]      
Leverage Ratio 2.75    
Credit Agreement | Citibank, National Association | Maximum | LIBOR rate      
Line of Credit Facility [Line Items]      
Interest rate 275.00%    
Credit Agreement | Citibank, National Association | Minimum      
Line of Credit Facility [Line Items]      
Interest Coverage Ratio 1.75    
Credit Agreement | Citibank, National Association | Minimum | LIBOR rate      
Line of Credit Facility [Line Items]      
Interest rate 175.00%