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v3.25.3

Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Accounting Policies [Line Items] |  |  |
Allowance at beginning of year | $ 31.4 | $ 32.5 | $ 31.9
Provision for expected credit losses | 19.6 | 23.0 | 16.6
Write-offs, net of recoveries | (22.2) | (24.1) | (16.0)
Allowance at end of year | 28.8 | 31.4 | 32.5
Spire Missouri [Member] |  |  |
Accounting Policies [Line Items] |  |  |
Allowance at beginning of year | 24.9 | 26.2 | 24.9
Provision for expected credit losses | 14.8 | 19.0 | 13.3
Write-offs, net of recoveries | (16.7) | (20.3) | (12.0)
Allowance at end of year | 23.0 | 24.9 | 26.2
Spire Alabama Inc [Member] |  |  |
Accounting Policies [Line Items] |  |  |
Allowance at beginning of year | 5.7 | 5.7 | 6.3
Provision for expected credit losses | 4.0 | 3.2 | 2.6
Write-offs, net of recoveries | (4.8) | (3.2) | (3.2)
Allowance at end of year | $ 4.9 | $ 5.7 | $ 5.7

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