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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Spire Missouri [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Unrecognized tax benefits, beginning of year | $ 25.3 | $ 22.9 | $ 19.3
Increases related to tax positions taken in current year | 0.2 | 2.4 | 3.6
Decreases related to tax positions taken in prior years | 24.2 | 0.0 | 0.0
Reductions due to lapse of applicable statute of limitations | 0.0 | 0.0 | 0.0
Unrecognized tax benefits, end of year | 1.3 | 25.3 | 22.9
Spire Alabama Inc [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Unrecognized tax benefits, beginning of year | 0.3 | 0.3 | 0.0
Increases related to tax positions taken in current year | 0.0 | 0.0 | 0.3
Decreases related to tax positions taken in prior years | 0.0 | 0.0 | 0.0
Reductions due to lapse of applicable statute of limitations | 0.0 | 0.0 | 0.0
Unrecognized tax benefits, end of year | 0.3 | 0.3 | 0.3
Parent Company [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Unrecognized tax benefits, beginning of year | 26.3 | 23.8 | 19.6
Increases related to tax positions taken in current year | 0.6 | 2.6 | 4.2
Decreases related to tax positions taken in prior years | (24.2) | 0.0 | 0.0
Reductions due to lapse of applicable statute of limitations | 0.0 | (0.1) | 0.0
Unrecognized tax benefits, end of year | $ 2.7 | $ 26.3 | $ 23.8

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