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Debt - Narrative (Details)
3 Months Ended 6 Months Ended
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2017
USD ($)
Debt Instrument [Line Items]            
Short-term borrowings   $ 9,300,000   $ 9,300,000   $ 0
Interest costs   7,900,000 $ 7,700,000 15,600,000 $ 15,300,000  
Interest costs, capitalized   600,000 $ 300,000 1,100,000 $ 500,000  
Credit Agreement            
Debt Instrument [Line Items]            
Maximum borrowing capacity $ 400,000,000          
Letters of credit issued   6,000,000   6,000,000    
Short-term borrowings   9,300,000   9,300,000    
Credit Agreement available for future borrowings   $ 384,700,000   $ 384,700,000    
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