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Debt - Narrative (Details)
3 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Debt Instrument [Line Items]    
Interest costs $ 7,200,000 $ 7,700,000
Interest costs, capitalized 900,000 $ 500,000
Credit Agreement    
Debt Instrument [Line Items]    
Maximum borrowing capacity 400,000,000  
Letters of credit issued 6,000,000  
Credit Agreement available for future borrowings $ 394,000,000  
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