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Shareholders' Equity - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Class of Stock [Line Items] |  |  |
Balance | $ 3,232.7 | $ 2,917.3 | $ 2,818.5
Other comprehensive (loss) income | 7.3 | (35.5) | 0.4
Balance | 3,389.4 | 3,232.7 | 2,917.3
Spire Missouri [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | 1,963.7 | 1,844.8 | 1,745.4
Other comprehensive (loss) income | 1.3 | 0.5 | 0.2
Balance | 2,167.7 | 1,963.7 | 1,844.8
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | 14.2 | 50.5 |
Other comprehensive (loss) income | 5.9 | (36.3) |
Balance | 20.1 | 14.2 | 50.5
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Spire Missouri [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | 0.0 | 0.0 |
Other comprehensive (loss) income | 0.0 | 0.0 |
Balance | 0.0 | 0.0 | 0.0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | (2.0) | (2.5) |
Other comprehensive (loss) income | 1.3 | 0.5 |
Balance | (0.7) | (2.0) | (2.5)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Spire Missouri [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | (2.0) | (2.5) |
Other comprehensive (loss) income | 1.3 | 0.5 |
Balance | (0.7) | (2.0) | (2.5)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | (0.1) | (0.4) |
Other comprehensive (loss) income | 0.1 | 0.3 |
Balance | 0.0 | (0.1) | (0.4)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] | Spire Missouri [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | 0.0 | 0.0 |
Other comprehensive (loss) income | 0.0 | 0.0 |
Balance | 0.0 | 0.0 | 0.0
AOCI Attributable to Parent [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | 12.1 | 47.6 | 47.2
Other comprehensive (loss) income | 7.3 | (35.5) | 0.4
Balance | 19.4 | 12.1 | 47.6
AOCI Attributable to Parent [Member] | Spire Missouri [Member] |  |  |
Class of Stock [Line Items] |  |  |
Balance | (2.0) | (2.5) | (2.7)
Other comprehensive (loss) income | 1.3 | 0.5 | 0.2
Balance | $ (0.7) | $ (2.0) | $ (2.5)

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