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Financial Instruments - Narrative (Details)
3 Months Ended 9 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Bcf
MMcf
Jun. 30, 2017
USD ($)
Derivative [Line Items]        
Net realized gain (loss) $ (48,700,000)   $ (48,700,000)  
Net gain (loss) on settled interest rate agreements $ (600,000) $ (200,000) $ (1,800,000) $ (500,000)
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts        
Derivative [Line Items]        
Hedging percent 26.00%   26.00%  
Energy measure | Bcf     15,000  
Minimum | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts        
Derivative [Line Items]        
Hedging percent 25.00%   25.00%  
Maximum | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts        
Derivative [Line Items]        
Hedging percent 50.00%   50.00%  
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap        
Derivative [Line Items]        
Notional amount $ 450,000,000   $ 450,000,000  
Interest rate hedged 3.78%   3.78%  
Long | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts        
Derivative [Line Items]        
Energy measure | MMcf     11,446