Note 11 - Commitments and Contingencies (Details Textual) $ in Millions	3 Months Ended
	Dec. 31, 2023 USD ($)
Long-Term Purchase Commitment, Amount	$ 1,900.3
Capital Committed to Contribute to Partnership	10.0
Remaining Unfunded Capital Commitment	4.9
Spire Missouri [Member]
Long-Term Purchase Commitment, Amount	1,331.4
Spire Alabama Inc [Member]
Long-Term Purchase Commitment, Amount	$ 403.5