Commitments and Contingencies - Additional Information - Commitments (Details) $ in Millions	3 Months Ended
	Dec. 31, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,554.6
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	690.3
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 239.3