Note 11 - Commitments and Contingencies (Details Textual) $ in Millions	6 Months Ended
	Mar. 31, 2024 USD ($)
Long-Term Purchase Commitment, Amount	$ 2,107.5
Capital Committed to Contribute to Partnership	10.0
Remaining Unfunded Capital Commitment	4.6
Spire Missouri [Member]
Long-Term Purchase Commitment, Amount	1,553.2
Spire Alabama Inc [Member]
Long-Term Purchase Commitment, Amount	$ 383.9