Commitments and Contingencies - Additional Information - Commitments (Details) $ in Millions	9 Months Ended
	Jun. 30, 2021 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,579.2
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	1,200.6
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 275.5