Commitments and Contingencies (Details) $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($) Bcf	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Long Term Purchase Commitment [Line Items]
Self-insured retention | $	$ 1.0
Purchases under contract | $	$ 58.5	$ 50.8	$ 57.2
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]
Purchase commitment volume	59,300
Contract term	1 year
Supply Commitment | Long-term Contract with Customer Within Two To Three Years
Long Term Purchase Commitment [Line Items]
Purchase commitment volume	57,000.0
Supply Commitment | Long-term Contract With Customer Beyond Three Years
Long Term Purchase Commitment [Line Items]
Purchase commitment volume	0.1
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