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
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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Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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-Publisher FASB
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Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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