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Summary of Significant Accounting Policies - Summary of Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Accounting Policies [Line Items] |  |  |
Accrued capital expenditures | $ 82.5 | $ 116.5 | $ 104.3
Spire Missouri [Member] |  |  |
Accounting Policies [Line Items] |  |  |
Accrued capital expenditures | 61.8 | 67.4 | 56.5
Spire Alabama Inc [Member] |  |  |
Accounting Policies [Line Items] |  |  |
Accrued capital expenditures | $ 9.5 | $ 14.1 | $ 4.6

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- Definition Future cash outflow to pay for purchases of fixed assets that have occurred. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-4 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-3 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-5 + Details
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- Details
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- Details
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