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Debt - Narrative (Details)
12 Months Ended
Mar. 31, 2017
USD ($)
Jun. 30, 2020
USD ($)
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Long-term debt outstanding        
Aggregate maturities of long-term debt, 2021   $ 0.0    
Aggregate maturities of long-term debt, 2022   250,000,000.0    
Aggregate maturities of long-term debt, 2023   300,000,000.0    
Aggregate maturities of long-term debt, 2024   0.0    
Aggregate maturities of long-term debt, 2025   0    
Interest costs   28,800,000 $ 31,100,000 $ 31,100,000
Interest costs, capitalized   $ 9,000,000.0 $ 5,100,000 $ 2,800,000
Credit Agreement        
Long-term debt outstanding        
Maximum borrowing capacity $ 400,000,000.0      
Commitment fee rate   0.20%    
Short-term credit agreement borrowings   $ 170,000,000.0    
Credit Agreement available for future borrowings   $ 224,000,000.0    
Borrowing rate   1.43%    
Required interest coverage ratio 3.50      
Credit Agreement | Minimum        
Long-term debt outstanding        
Commitment fee rate 0.125%      
Credit Agreement | Maximum        
Long-term debt outstanding        
Commitment fee rate 0.40%      
Required debt to capital ratio 0.55      
Credit Agreement | LIBOR        
Long-term debt outstanding        
Interest rate margin   1.25%    
Credit Agreement | LIBOR | Minimum        
Long-term debt outstanding        
Interest rate margin 1.00%      
Credit Agreement | LIBOR | Maximum        
Long-term debt outstanding        
Interest rate margin 1.75%      
Credit Agreement | Base Rate        
Long-term debt outstanding        
Interest rate margin   0.25%    
Credit Agreement | Base Rate | Minimum        
Long-term debt outstanding        
Interest rate margin 0.00%      
Credit Agreement | Base Rate | Maximum        
Long-term debt outstanding        
Interest rate margin 0.75%      
Credit Agreement | Letter of Credit        
Long-term debt outstanding        
Letter of credit fees   1.50%    
Letters of credit issued   $ 6,000,000.0    
Credit Agreement | Letter of Credit | Minimum        
Long-term debt outstanding        
Letter of credit fees 1.00%      
Credit Agreement | Letter of Credit | Maximum        
Long-term debt outstanding        
Letter of credit fees 1.75%