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Long-term Debt - Additional Information (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Mar. 24, 2020
Debt Instrument [Line Items] |  |  |  |
Long-term debt | $ 2,500,000,000.0 | $ 2,137,000,000.0 |  |
Interest expense, net | 5,800,000 | 6,800,000 | $ 2,600,000 |
Spire Missouri |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt | 1,098,000,000.0 | 930,000,000.0 |  |
Interest expense, net | 800,000 | 1,900,000 | $ 900,000 |
Regulatory preferred stock and debt securities issuance, amount authorized | 500,000,000.0 |  |  |
Regulatory preferred stock and debt securities issuance, remaining authorized amount | 125,000,000.0 |  |  |
Spire Alabama |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt | 475,000,000.0 | $ 415,000,000.0 |  |
Interest expense, net | 1,900,000 |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized |  |  |  | $ 150,000,000.0
Subsidiaries |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt | $ 237,000,000.0 |  |  |

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