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Commitments and Contingencies - Additional Information - Commitments (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019
Long-term Purchase Commitment [Line Items] |  |
Minimum total payments required for natural gas contracts | $ 1,745.4 |
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 | 1,410.3 |
Spire Missouri | Purchase Commitment | Subsidiary of Common Parent |  |
Long-term Purchase Commitment [Line Items] |  |
Annual purchase commitment | $ 1.0 |
Commitment termination date | Mar. 31, 2021 |
Spire Alabama |  |
Long-term Purchase Commitment [Line Items] |  |
Minimum total payments required for natural gas contracts | $ 229.7 |

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