Derivative Instruments and Hedging Activities - Exchange-Traded/Cleared Derivatives (Details) - MMBTU MMBTU in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Natural Gas Futures Purchased | Gas Marketing
Derivative [Line Items]
Notional (MMBtu millions)	22.9	29.1
Maximum Term (Months)	41 months	36 months
Natural Gas Futures Purchased | Gas Utility
Derivative [Line Items]
Notional (MMBtu millions)	25.9	69.0
Maximum Term (Months)	12 months	12 months
Natural Gas Options Purchased Net | Gas Marketing
Derivative [Line Items]
Notional (MMBtu millions)	4.8	6.9
Maximum Term (Months)	6 months	12 months
Natural Gas Basis Swaps Purchased | Gas Marketing
Derivative [Line Items]
Notional (MMBtu millions)	6.2	90.1
Maximum Term (Months)	24 months	36 months