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Financial Instruments - Narrative (Details)
$ in Millions
12 Months Ended
Sep. 30, 2019
USD ($)
$ / Mcf
Bcf
MMcf
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2015
USD ($)
Derivative [Line Items]        
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense $ 3.9 $ 2.4 $ 1.0  
AOCI impact from settlement of financial instruments $ (114.7)      
Interest Rate Hedges, Senior Notes 2019 Issuance        
Derivative [Line Items]        
Derivative, Notional Amount       $ 450.0
Anticipated Debt Issuance, Amount       $ 450.0
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts        
Derivative [Line Items]        
Hedging percent 33.00%      
Purchase commitment volume | Bcf 18,900      
Weighted average cost | $ / Mcf 2.86      
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Minimum        
Derivative [Line Items]        
Hedging percent 25.00%      
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Maximum        
Derivative [Line Items]        
Hedging percent 50.00%      
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Long        
Derivative [Line Items]        
Purchase commitment volume | MMcf 24,270      
Designated as Hedging Instrument | Interest Rate Swap | Cash Flow Hedging        
Derivative [Line Items]        
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense $ 90.1