Note 16 - Commitments and Contingencies (Details Textual) $ in Millions	12 Months Ended
	Sep. 30, 2024 USD ($)
Long-Term Purchase Commitment, Amount	$ 1,529.5
Capital Committed to Contribute to Partnership	25.3
Remaining Unfunded Capital Commitment	18.4
Spire Missouri [Member]
Long-Term Purchase Commitment, Amount	1,493.1
Spire Alabama Inc [Member]
Long-Term Purchase Commitment, Amount	$ 346.8