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Debt - Narrative (Details)
3 Months Ended 6 Months Ended
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jun. 30, 2016
USD ($)
Debt Instrument [Line Items]          
Short-term borrowings $ 25,000,000   $ 25,000,000   $ 0
Interest costs 7,700,000 $ 7,500,000 15,300,000 $ 14,700,000  
Interest costs, capitalized 300,000 $ 500,000 500,000 $ 1,100,000  
Credit Agreement          
Debt Instrument [Line Items]          
Maximum borrowing capacity 500,000,000   500,000,000    
Letters of credit issued 5,800,000   5,800,000    
Short-term borrowings 25,000,000   25,000,000    
Credit Agreement available for future borrowings $ 469,200,000   $ 469,200,000    
Credit Agreement | LIBOR          
Debt Instrument [Line Items]          
Variable rate basis     LIBOR    
Interest rate margin     1.45%    
Credit Agreement | LIBOR | Minimum          
Debt Instrument [Line Items]          
Interest rate margin     0.75%    
Credit Agreement | LIBOR | Maximum          
Debt Instrument [Line Items]          
Interest rate margin     1.90%    
Credit Agreement | Base Rate          
Debt Instrument [Line Items]          
Variable rate basis     Base Rate    
Interest rate margin     0.45%    
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     0.90%    
Letters of credit          
Debt Instrument [Line Items]          
Commitment fee rate     0.225%    
Letter of credit fees     1.45%    
Required interest coverage ratio     3.50    
Letters of credit | Minimum          
Debt Instrument [Line Items]          
Commitment fee rate     0.075%    
Letter of credit fees     0.75%    
Letters of credit | Maximum          
Debt Instrument [Line Items]          
Commitment fee rate     0.375%    
Letter of credit fees     1.90%    
Required debt to capital ratio (less than 55%) 0.55   0.55