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Debt - Narrative (Details)
3 Months Ended 9 Months Ended
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Jun. 30, 2018
USD ($)
Debt Instrument [Line Items]          
Short-term borrowings $ 127,000,000   $ 127,000,000   $ 0
Interest costs 8,400,000 $ 8,200,000 23,500,000 $ 23,800,000  
Interest costs, capitalized 1,300,000 $ 700,000 3,200,000 $ 1,800,000  
Credit Agreement          
Debt Instrument [Line Items]          
Maximum borrowing capacity 400,000,000   400,000,000    
Letters of credit issued 6,000,000   6,000,000    
Short-term borrowings 127,000,000   127,000,000    
Credit Agreement available for future borrowings $ 267,000,000   $ 267,000,000    
Credit Agreement | LIBOR          
Debt Instrument [Line Items]          
Interest rate margin     1.25%    
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     0.25%    
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     0.20%    
Letter of credit fees     1.25%    
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) 0.55   0.55