Note 10 - Derivative Instruments and Hedging Activities (Details Textual) $ in Thousands	12 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($) MMBoe	Sep. 30, 2022 USD ($)
Nonmonetary Notional Amount of Natural Gas Derivative Contracts Not Designated As Normal Purchases Or Normal Sales (Millions of Barrels of Oil Equivalent)		567,000,000
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Next Twelve Months (Millions of Barrels of Oil Equivalent)		379.1
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Two (Millions of Barrels of Oil Equivalent)		151.1
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Three (Millions of Barrels of Oil Equivalent)		26.4
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Four (Millions of Barrels of Oil Equivalent)		3.8
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Five (Millions of Barrels of Oil Equivalent)		6.6
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax | $		$ 44,200
Other Receivable from Broker-Dealer and Clearing Organization | $		$ 0	$ 49,800
Forecast [Member]
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent | $	$ 2,800
Spire Missouri [Member]
Nonmonetary Notional Amount of Other Price Mitigation in Place Not Designated As Normal Purchases Or Normal Sales (Millions of Barrels of Oil Equivalent)		16,300,000
Other Receivable from Broker-Dealer and Clearing Organization | $		$ 0	$ 24,000