Note 10 - Derivative Instruments and Hedging Activities - Summary of NYMEX and ICE Natural Gas Futures and Swap Positions (Details) - MMBTU MMBTU in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Natural Gas Futures Purchased [Member] | Spire Missouri [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	34.6	66.5
Derivative, hedge term (Year)	12 months	24 months
Natural Gas Futures Purchased [Member] | Gas Marketing [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	56.0	61.3
Derivative, hedge term (Year)	48 months	51 months
Natural Gas Futures Purchased [Member] | Gas Utility [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	34.6	66.5
Derivative, hedge term (Year)	12 months	24 months
Natural Gas Options Purchased Net [Member] | Gas Marketing [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	3.8	4.9
Derivative, hedge term (Year)	15 months	15 months
Natural Gas Basis Swaps Purchased [Member] | Gas Marketing [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	32.4	44.1
Derivative, hedge term (Year)	39 months	51 months