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Pension Plans and Other Postretirement Benefits - Schedule of Projected Benefit Obligation, Accumulated Benefit Obligation, and Fair Value of Plan Assets (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
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Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | $ 732.6 | $ 751.4
Accumulated benefit obligation | 699.3 | 712.9
Fair value of plan assets | 473.1 | 521.8
Spire Missouri |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | 505.2 | 538.4
Accumulated benefit obligation | 473.7 | 500.2
Fair value of plan assets | 336.2 | 379.2
Spire Alabama |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | 163.5 | 152.5
Accumulated benefit obligation | 161.8 | 152.3
Fair value of plan assets | $ 88.6 | $ 93.3

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