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Segment Information - Schedule of Financial Data Related to Gross Receipt Taxes and Capital Expenditures (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Property, Plant and Equipment [Line Items] |  |  |
Capital Expenditures | $ 922.4 | $ 861.3 | $ 662.5
Spire Missouri [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Gross Receipt Taxes | 82.9 | 93.1 | 96.7
Capital Expenditures | 641.2 | 553.0 | 447.5
Spire Alabama Inc [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Gross Receipt Taxes | 27.9 | 30.1 | 29.9
Capital Expenditures | $ 144.2 | $ 112.8 | $ 117.6

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- Definition The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 13 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-13 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478964/842-20-50-7A + Details
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