Derivative Instruments and Hedging Activities - Additional Information (Details) MMBTU in Thousands, $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) MMBTU
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Nonmonetary Notional Amount of Natural Gas Derivative Contracts Not Designated As Normal Purchases Or Normal Sales (Millions of Barrels of Oil Equivalent)	278,200
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Next Twelve Months (Millions of Barrels of Oil Equivalent)	236,700
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Two (Millions of Barrels of Oil Equivalent)	29,700
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Three (Millions of Barrels of Oil Equivalent)	8,200
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Four (Millions of Barrels of Oil Equivalent)	3,400
Notional Amount of Non Exchange Traded Natural Gas Commodity Contracts To Be Settled In Year Five (Millions of Barrels of Oil Equivalent)	200
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax | $	$ 10.0
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent | $	$ 2.6
Spire Missouri [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Nonmonetary Notional Amount of Other Price Mitigation in Place Not Designated As Normal Purchases Or Normal Sales (Millions of Barrels of Oil Equivalent)	9,780