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Note 13 - 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, 2023 | Sep. 30, 2022
--- | --- | ---
Projected benefit obligation | $ 438.7 | $ 464.4
Accumulated benefit obligation | 433.1 | 457.1
Fair value of plan assets | 334.7 | 332.7
Spire Missouri [Member] |  |
Projected benefit obligation | 313.2 | 320.6
Accumulated benefit obligation | 309.5 | 315.0
Fair value of plan assets | 242.0 | 235.9
Spire Alabama Inc [Member] |  |
Projected benefit obligation | 80.6 | 97.8
Accumulated benefit obligation | 78.8 | 96.2
Fair value of plan assets | $ 53.6 | $ 57.3

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