Commitments and Contingencies - Additional Information - Commitments (Details) $ in Millions	6 Months Ended
	Mar. 31, 2021 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,632.2
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	1,264.6
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 244.7