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Commitments and Contingencies (Details)
$ in Millions
12 Months Ended
Sep. 30, 2019
USD ($)
Bcf
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Long Term Purchase Commitment [Line Items]      
Self-insured retention | $ $ 1.0    
Purchases under contract | $ $ 50.8 $ 57.2 $ 49.7
Supply Commitment      
Long Term Purchase Commitment [Line Items]      
Contract term 1 year    
Supply Commitment | Short-term Contract with Customer      
Long Term Purchase Commitment [Line Items]      
Contract term 1 year    
Purchase commitment volume | Bcf 40,100    
Supply Commitment | Long-term Contract with Customer Within Two To Three Years      
Long Term Purchase Commitment [Line Items]      
Purchase commitment volume | Bcf 1,600    
Minimum | Supply Commitment | Long-term Contract with Customer Within Two To Three Years      
Long Term Purchase Commitment [Line Items]      
Contract term 2 years    
Maximum | Supply Commitment | Long-term Contract with Customer Within Two To Three Years      
Long Term Purchase Commitment [Line Items]      
Contract term 3 years