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Note 5 - Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Balance |  | $ 2,818.5 |  | $ 2,658.2 |  | $ 2,522.3 |
Other Comprehensive Income (Loss), Net of Tax |  | 0.4 |  | 43.6 |  | 44.8 |
Balance |  | 2,917.3 |  | 2,818.5 |  | 2,658.2 |
Spire Missouri [Member] |  |  |  |  |  |  |
Balance | [1] | 1,745.4 |  | 1,577.9 |  | 1,435.1 |
Other Comprehensive Income (Loss), Net of Tax |  | 0.2 | [1] | 1.5 | [1] | (1.3) |
Balance | [1] | 1,844.8 |  | 1,745.4 |  | 1,577.9 |
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] |  |  |  |  |  |  |
Balance |  | 50.4 |  | 7.9 |  |  |
Other Comprehensive Income (Loss), Net of Tax |  | 0.1 |  | 42.5 |  |  |
Balance |  | 50.5 |  | 50.4 |  | 7.9 |
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Spire Missouri [Member] |  |  |  |  |  |  |
Balance |  | 0.0 |  | 0.0 |  |  |
Other Comprehensive Income (Loss), Net of Tax |  | 0.0 |  | 0.0 |  |  |
Balance |  | 0.0 |  | 0.0 |  | 0.0 |
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] |  |  |  |  |  |  |
Balance |  | (2.7) |  | (4.2) |  |  |
Other Comprehensive Income (Loss), Net of Tax |  | 0.2 |  | 1.5 |  |  |
Balance |  | (2.5) |  | (2.7) |  | (4.2) |
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Spire Missouri [Member] |  |  |  |  |  |  |
Balance |  | (2.7) |  | (4.2) |  |  |
Other Comprehensive Income (Loss), Net of Tax |  | 0.2 |  | 1.5 |  |  |
Balance |  | (2.5) |  | (2.7) |  | (4.2) |
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] |  |  |  |  |  |  |
Balance |  | (0.5) |  | (0.1) |  |  |
Other Comprehensive Income (Loss), Net of Tax |  | 0.1 |  | (0.4) |  |  |
Balance |  | (0.4) |  | (0.5) |  | (0.1) |
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] | Spire Missouri [Member] |  |  |  |  |  |  |
Balance |  | 0.0 |  | 0.0 |  |  |
Other Comprehensive Income (Loss), Net of Tax |  | 0.0 |  | 0.0 |  |  |
Balance |  | 0.0 |  | 0.0 |  | 0.0 |
AOCI Attributable to Parent [Member] |  |  |  |  |  |  |
Balance |  | 47.2 |  | 3.6 | [1] | (41.2) | [1]
Other Comprehensive Income (Loss), Net of Tax |  | 0.4 |  | 43.6 |  | 44.8 |
Balance |  | 47.6 |  | 47.2 |  | 3.6 | [1]
AOCI Attributable to Parent [Member] | Spire Missouri [Member] |  |  |  |  |  |  |
Balance |  | (2.7) |  | (4.2) |  | (2.9) |
Other Comprehensive Income (Loss), Net of Tax |  | 0.2 |  | 1.5 |  | (1.3) |
Balance |  | $ (2.5) |  | $ (2.7) |  | $ (4.2) |
[1] | Accumulated other comprehensive income (loss)

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