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Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
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Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | $ 78.3 | $ 115.4
Regulatory Assets, Noncurrent | 1,323.5 | 1,251.8
Total Regulatory Assets | 1,401.8 | 1,367.2
Regulatory Liabilities, Current | 39.4 | 49.5
Regulatory Liabilities, Noncurrent | 578.0 | 535.5
Total Regulatory Liabilities | 617.4 | 585.0
Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 48.9 | 84.0
Regulatory Assets, Noncurrent | 653.2 | 588.0
Total Regulatory Assets | 702.1 | 672.0
Regulatory Liabilities, Current | 13.9 | 10.2
Regulatory Liabilities, Noncurrent | 481.3 | 443.3
Total Regulatory Liabilities | 495.2 | 453.5
Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 16.1 | 19.2
Regulatory Assets, Noncurrent | 650.6 | 642.0
Total Regulatory Assets | 666.7 | 661.2
Regulatory Liabilities, Current | 20.3 | 33.8
Regulatory Liabilities, Noncurrent | 29.2 | 28.3
Total Regulatory Liabilities | 49.5 | 62.1
Future Income Taxes Due from Customers [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 154.2 | 150.7
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 146.6 | 142.7
Future Income Taxes Due from Customers [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 1.6 | 1.8
Deferred Taxes Due to Customers [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Liabilities, Noncurrent | 100.3 | 114.2
Deferred Taxes Due to Customers [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Liabilities, Noncurrent | 88.8 | 101.8
Deferred Taxes Due to Customers [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Liabilities, Noncurrent | 0.0 | 0.0
Pension and Postretirement Benefit Costs [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 217.8 | 237.5
Regulatory Liabilities, Noncurrent | 300.1 | 232.9
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 157.0 | 166.5
Regulatory Liabilities, Noncurrent | 261.2 | 196.6
Pension and Postretirement Benefit Costs [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 59.1 | 68.2
Regulatory Liabilities, Noncurrent | 26.5 | 25.1
Cost of Removal [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 688.3 | 668.2
Regulatory Liabilities, Noncurrent | 138.2 | 133.6
Cost of Removal [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 98.8 | 97.0
Regulatory Liabilities, Noncurrent | 96.0 | 94.5
Cost of Removal [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 589.6 | 571.2
Regulatory Liabilities, Noncurrent | 0.0 | 0.0
Unamortized Purchased Gas Adjustments [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 16.5 | 28.9
Regulatory Assets, Noncurrent | 9.0 | 1.2
Regulatory Liabilities, Current | 34.9 | 42.3
Regulatory Liabilities, Noncurrent | 6.5 | 17.2
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 14.9 | 28.1
Regulatory Assets, Noncurrent | 9.0 | 1.2
Regulatory Liabilities, Current | 13.9 | 10.2
Regulatory Liabilities, Noncurrent | 6.5 | 17.2
Unamortized Purchased Gas Adjustments [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 0.0 | 0.0
Regulatory Assets, Noncurrent | 0.0 | 0.0
Regulatory Liabilities, Current | 20.2 | 30.9
Regulatory Liabilities, Noncurrent | 0.0 | 0.0
Energy Efficiency [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 65.0 | 61.0
Energy Efficiency [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 65.0 | 61.0
Energy Efficiency [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Noncurrent | 0.0 | 0.0
Other [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 61.8 | 86.5
Regulatory Assets, Noncurrent | 189.2 | 133.2
Regulatory Liabilities, Current | 4.5 | 7.2
Regulatory Liabilities, Noncurrent | 32.9 | 37.6
Other [Member] | Spire Missouri [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 34.0 | 55.9
Regulatory Assets, Noncurrent | 176.8 | 119.6
Regulatory Liabilities, Current | 0.0 | 0.0
Regulatory Liabilities, Noncurrent | 28.8 | 33.2
Other [Member] | Spire Alabama Inc [Member] |  |
Schedule Of Regulatory Assets and Liabilities [Line Items] |  |
Regulatory Assets, Current | 16.1 | 19.2
Regulatory Assets, Noncurrent | 0.3 | 0.8
Regulatory Liabilities, Current | 0.1 | 2.9
Regulatory Liabilities, Noncurrent | $ 2.7 | $ 3.2

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + 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 + Details
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