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, 2023	Sep. 30, 2022
Natural Gas Futures Purchased [Member] | Spire Missouri [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	66.5	13.0
Derivative, hedge term (Year)	24 months	12 months
Natural Gas Futures Purchased [Member] | Gas Marketing [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	61.3	76.3
Derivative, hedge term (Year)	51 months	54 months
Natural Gas Futures Purchased [Member] | Gas Utility [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	66.5	13.0
Derivative, hedge term (Year)	24 months	12 months
Natural Gas Options Purchased Net [Member] | Gas Marketing [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	4.9	3.7
Derivative, hedge term (Year)	15 months	12 months
Natural Gas Basis Swaps Purchased [Member] | Gas Marketing [Member]
Notional Amount, Energy Measure (Millions of British Thermal Unit)	44.1	61.7
Derivative, hedge term (Year)	51 months	39 months