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Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
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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

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- 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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