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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

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 715
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-Name Accounting Standards Codification
-Section 55
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-URI https://asc.fasb.org/1943274/2147480482/715-20-55-17

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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-Section 55
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-URI https://asc.fasb.org/1943274/2147480482/715-20-55-17

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+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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