Derivative Instruments and Hedging Activities - Exchange-Traded/Cleared Derivatives (Details) - MMBTU MMBTU in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Natural Gas Futures Purchased | Gas Marketing
Derivative [Line Items]
Notional (MMBtu millions)	103.3	22.9
Maximum Term (Months)	51 months	41 months
Natural Gas Futures Purchased | Gas Utility
Derivative [Line Items]
Notional (MMBtu millions)	52.8	25.9
Maximum Term (Months)	12 months	12 months
Natural Gas Options Purchased Net | Gas Marketing
Derivative [Line Items]
Notional (MMBtu millions)	7.1	4.8
Maximum Term (Months)	15 months	6 months
Natural Gas Basis Swaps Purchased | Gas Marketing
Derivative [Line Items]
Notional (MMBtu millions)	101.7	6.2
Maximum Term (Months)	27 months	24 months