Commitments and Contingencies - Additional Information - Commitments (Details) $ in Millions	6 Months Ended
	Mar. 31, 2020 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,558.7
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	760.3
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 189.2