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Debt - Narrative (Details)
12 Months Ended
Mar. 31, 2017
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2018
Long-term debt outstanding          
Aggregate maturities of long-term debt, 2019   $ 0      
Aggregate maturities of long-term debt, 2020   0      
Aggregate maturities of long-term debt, 2021   0      
Aggregate maturities of long-term debt, 2022   250,000,000      
Aggregate maturities of long-term debt, 2023   300,000,000      
Interest costs   31,100,000 $ 31,100,000 $ 29,900,000  
Interest costs, capitalized   2,800,000 $ 1,300,000 $ 1,900,000  
Credit Agreement          
Long-term debt outstanding          
Maximum borrowing capacity $ 400,000,000.0        
Letters of credit issued   6,000,000      
Credit Agreement available for future borrowings   $ 394,000,000      
Credit Agreement | LIBOR          
Long-term debt outstanding          
Interest rate margin   1.50%      
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   0.50%      
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%        
Letter of Credit          
Long-term debt outstanding          
Commitment fee rate   0.275%      
Letter of credit fees   1.50%      
Required interest coverage ratio 3.50        
Letter of Credit | Minimum          
Long-term debt outstanding          
Commitment fee rate 0.125%        
Letter of credit fees 1.00%        
Letter of Credit | Maximum          
Long-term debt outstanding          
Commitment fee rate 0.40%        
Letter of credit fees 1.75%        
Required debt to capital ratio         0.55