Commitments and Contingencies - Additional Information - Commitments (Details) $ in Millions	12 Months Ended
	Sep. 30, 2021 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,889.0
Committed Capital to Contribute	10.0
Contributed Capital	0.3
Outstanding mortgage loans incurred in connection with various real estate ventures with general partners	1.2
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	1,414.2
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 363.0