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Debt - Narrative (Details)
3 Months Ended
Mar. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Debt Instrument [Line Items]        
Short-term borrowings   $ 3,300,000   $ 0
Interest costs   7,700,000 $ 7,500,000  
Interest costs, capitalized   500,000 $ 200,000  
Credit Agreement        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 400,000,000      
Letters of credit issued   6,100,000    
Short-term borrowings   3,300,000    
Credit Agreement available for future borrowings   $ 390,600,000    
Credit Agreement | LIBOR        
Debt Instrument [Line Items]        
Interest rate margin   1.50%    
Credit Agreement | LIBOR | Minimum        
Debt Instrument [Line Items]        
Interest rate margin 1.00%      
Credit Agreement | LIBOR | Maximum        
Debt Instrument [Line Items]        
Interest rate margin 1.75%      
Credit Agreement | Base Rate        
Debt Instrument [Line Items]        
Interest rate margin   0.50%    
Credit Agreement | Base Rate | Minimum        
Debt Instrument [Line Items]        
Interest rate margin 0.00%      
Credit Agreement | Base Rate | Maximum        
Debt Instrument [Line Items]        
Interest rate margin 0.75%      
Letters of credit        
Debt Instrument [Line Items]        
Commitment fee rate   0.275%    
Letter of credit fees   1.50%    
Required interest coverage ratio 3.50      
Letters of credit | Minimum        
Debt Instrument [Line Items]        
Commitment fee rate 0.125%      
Letter of credit fees 1.00%      
Letters of credit | Maximum        
Debt Instrument [Line Items]        
Commitment fee rate 0.40%      
Letter of credit fees 1.75%      
Required debt to capital ratio (less than 55%) 0.55