Quarterly report pursuant to Section 13 or 15(d)

Debt and Credit Arrangements - Narrative (Details)

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Debt and Credit Arrangements - Narrative (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Line of Credit Facility [Line Items]        
Revolving credit facility $ 154,235,000 $ 154,235,000   $ 140,000,000
Revolving credit facility | Line of Credit        
Line of Credit Facility [Line Items]        
Revolving credit facility 155,000,000 155,000,000   $ 140,000,000
Credit Agreement | Citibank, National Association        
Line of Credit Facility [Line Items]        
Aggregate secured revolving credit facility 225,000,000.0 $ 225,000,000.0 $ 150,000,000  
Increase in credit facility $ 75,000,000      
Leverage Ratio 2.29 2.29    
Interest Coverage Ratio 14.28 14.28    
All-in rate 3.49% 3.49%    
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 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 1.75    
Credit Agreement | Citibank, National Association | Minimum | LIBOR rate        
Line of Credit Facility [Line Items]        
Interest rate   175.00%