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Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Regulatory assets not earning a return	$ 472.7	$ 440.4
Spire Missouri [Member]
Regulatory assets not earning a return	466.6	434.2
Pension and Postretirement Benefit Costs [Member]
Regulatory assets not earning a return	119.1	129.7
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	119.1	129.7
Future Income Taxes Due from Customers [Member]
Regulatory assets not earning a return	152.7	148.9
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	146.6	142.7
Unamortized Purchased Gas Adjustments [Member]
Regulatory assets not earning a return	23.9	29.3
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	23.9	29.3
Other [Member]
Regulatory assets not earning a return	177.0	132.5
Other [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 177.0	$ 132.5

X

- Definition

Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478742/980-340-50-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 55
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478098/980-340-55-2

+ Details

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