Note 10 - Derivative Instruments and Hedging Activities (Details Textual) MMBoe in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($) MMBoe
Nonmonetary Notional Amount of Natural Gas Derivative Contracts Not Designated As Normal Purchases Or Normal Sales (Millions of Barrels of Oil Equivalent)		450.3
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Next Twelve Months (Millions of Barrels of Oil Equivalent)		387.7
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Two (Millions of Barrels of Oil Equivalent)		49.7
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Three (Millions of Barrels of Oil Equivalent)		6.3
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)		2.8
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax | $		$ 0.7
Forecast [Member]
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent | $	$ 2.6
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)		13.0