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Note 5 - Shareholders' Equity (Tables) | 12 Months Ended
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Sep. 30, 2023
Notes Tables |
Stock Purchase Contract Obligation, Number of Shares to be Purchased [Table Text Block] |
If the applicable market value* per share |  | Number of shares to be purchased per
of Spire common stock is: |  | stock purchase contract is:
Equal to or greater than $78.6906 (“threshold appreciation price”) |  | 0.6354 (“minimum settlement rate”)
Less than $78.6906, but greater than $64.24 |  | $50.00 ÷ applicable market value*
Less than or equal to $64.24 (“reference price”) |  | 0.7783 (“maximum settlement rate”)
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] |
|  | Net |  |  | Defined Benefit |  |  | Net Unrealized |  |  |  |  |
|  | Unrealized |  |  | Pension and |  |  | Gain (Loss) on |  |  |  |  |
|  | Gain (Loss) |  |  | Other |  |  | Available-for- |  |  |  |  |
|  | on Cash Flow |  |  | Postretirement |  |  | Sale Debt |  |  |  |  |
|  | Hedges |  |  | Benefit Plans |  |  | Securities |  |  | Total |
Spire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance at September 30, 2021 |  | $ | 7.9 |  |  | $ | (4.2 | ) |  | $ | (0.1 | ) |  | $ | 3.6 |
Other comprehensive income (loss) |  |  | 42.5 |  |  |  | 1.5 |  |  |  | (0.4 | ) |  |  | 43.6 |
Balance at September 30, 2022 |  |  | 50.4 |  |  |  | (2.7 | ) |  |  | (0.5 | ) |  |  | 47.2 |
Other comprehensive income |  |  | 0.1 |  |  |  | 0.2 |  |  |  | 0.1 |  |  |  | 0.4 |
Balance at September 30, 2023 |  | $ | 50.5 |  |  | $ | (2.5 | ) |  | $ | (0.4 | ) |  | $ | 47.6 |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance at September 30, 2021 |  | $ | — |  |  | $ | (4.2 | ) |  | $ | — |  |  | $ | (4.2 | )
Other comprehensive income |  |  | — |  |  |  | 1.5 |  |  |  | — |  |  |  | 1.5 |
Balance at September 30, 2022 |  |  | — |  |  |  | (2.7 | ) |  |  | — |  |  |  | (2.7 | )
Other comprehensive income |  |  | — |  |  |  | 0.2 |  |  |  | — |  |  |  | 0.2 |
Balance at September 30, 2023 |  | $ | — |  |  | $ | (2.5 | ) |  | $ | — |  |  | $ | (2.5 | )

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