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Leases - Schedule of Operating Lease Cost, Cash Flow and Noncash Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Operating lease cost, including amounts capitalized | $ 7.1 | $ 7.3 | $ 7.3
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities | 7.3 | 7.4 | 7.3
Right-of-use assets obtained in exchange for lease liabilities | 0.0 | 0.0 | 0.8
Spire Missouri [Member] |  |  |
Operating lease cost, including amounts capitalized | 0.5 | 0.5 | 0.5
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities | 0.5 | 0.5 | 0.5
Right-of-use assets obtained in exchange for lease liabilities | 0.0 | 0.0 | 0.8
Spire Alabama Inc [Member] |  |  |
Operating lease cost, including amounts capitalized | 2.0 | 2.0 | 2.0
Operating cash flows representing cash paid for amounts included in the measurement of lease liabilities | 2.2 | 2.1 | 2.1
Right-of-use assets obtained in exchange for lease liabilities | $ 0.0 | $ 0.0 | $ 0.0

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- Definition Amount of single lease cost, calculated by allocation of remaining cost of lease over remaining lease term. Includes, but is not limited to, single lease cost, after impairment of right-of-use asset, calculated by amortization of remaining right-of-use asset and accretion of lease liability. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 53 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479589/842-20-55-53 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478964/842-20-50-4 + Details
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- Definition Amount of cash outflow from operating lease, excluding payments to bring another asset to condition and location necessary for its intended use. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 5 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (g)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478964/842-20-50-4 + Details
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- Definition Amount of increase in right-of-use asset obtained in exchange for operating lease liability. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 53 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479589/842-20-55-53 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (g)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478964/842-20-50-4 + Details
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- Details
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