Commitments and Contingencies - Additional Information (Details) $ in Millions	6 Months Ended
	Mar. 31, 2025 USD ($)
Long-Term Purchase Commitment [Line Items]
Long-term purchase commitment, amount	$ 1,642.2
Capital committed to contribute to partnership	25.3
Remaining unfunded capital commitment	15.6
Spire Missouri [Member]
Long-Term Purchase Commitment [Line Items]
Long-term purchase commitment, amount	1,513.4
Spire Alabama Inc [Member]
Long-Term Purchase Commitment [Line Items]
Long-term purchase commitment, amount	$ 387.0