Commitments and Contingencies - Additional Information (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($)
Long-Term Purchase Commitment [Line Items]
Long-term purchase commitment, amount	$ 1,762.8
Capital committed to contribute to partnership	25.0
Remaining unfunded capital commitment	15.4
Spire Missouri [Member]
Long-Term Purchase Commitment [Line Items]
Long-term purchase commitment, amount	1,388.9
Spire Alabama Inc [Member]
Long-Term Purchase Commitment [Line Items]
Long-term purchase commitment, amount	$ 557.2