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v3.25.3

Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2023 USD ($) | Sep. 30, 2025 USD ($) Segment | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($)
Accounting Policies [Line Items] |  |  |  |
Number of reportable segments | Segment |  | 3 |  |
Difference between replacement cost of gas storage and inventory basis amount |  | $ 21.3 | $ 27.3 |
Goodwill |  | 1,171.6 | 1,171.6 |
Restricted Cash |  | 35.5 | 30.4 |
Utility Plant [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Utility plant, construction work-in-progress |  | 268.3 | 143.2 |
Non-utility Property [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Depreciation |  | $ 32.7 | $ 25.8 | $ 20.6
Spire Alabama Inc [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Number of reportable segments | Segment |  | 1 |  |
Public utilities, property, plant and equipment, disclosure of composite depreciation rate for plants in service |  | 3.00% | 3.00% | 3.00%
Financing Receivable, after Allowance for Credit Loss |  | $ 5.4 | $ 5.9 |
Gas Utility [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Goodwill |  | 210.2 |  |
Gas Marketing [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Goodwill |  | 0.0 |  |
Midstream [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Goodwill |  | 0.0 |  |
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Spire Alabama Inc [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Financing Receivable, before Allowance for Credit Loss |  | $ 0.4 | $ 0.4 |
Other Income [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Gain on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring | $ 8.2 |  |  |

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