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Pension Plans and Other Postretirement Benefits - PBO, ABO, and Fair Value of Plan Assets for Plans in Excess of Plan Assets (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Projected benefit obligation | $ 450.2 | $ 469.7
Accumulated benefit obligation | 439.6 | 459.4
Fair value of plan assets | 427.7 | 374.8
Spire Missouri [Member] |  |
Projected benefit obligation | 307.2 | 319.7
Accumulated benefit obligation | 302.8 | 314.7
Fair value of plan assets | 284.8 | 259.8
Spire Alabama Inc [Member] |  |
Projected benefit obligation | 93.0 | 97.9
Accumulated benefit obligation | 87.3 | 92.9
Fair value of plan assets | $ 90.6 | $ 69.9

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