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Fair Value of Financial Instruments - Schedule of Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Reported Value Measurement [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | $ 5.7 | $ 4.5
Notes payable | 1,317.0 | 947.0
Long-term debt, including current portion | 3,856.9 | 3,746.4
Reported Value Measurement [Member] | Spire Missouri [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes payable - associated companies | 566.3 | 495.3
Long-term debt, including current portion | 1,953.6 | 1,803.4
Reported Value Measurement [Member] | Spire Alabama Inc [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 1.9 | 1.5
Notes payable - associated companies | 130.1 | 48.4
Long-term debt, including current portion | 711.7 | 746.3
Estimate of Fair Value Measurement [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 5.7 | 4.5
Notes payable | 1,317.0 | 947.0
Long-term debt, including current portion | 3,691.5 | 3,600.3
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes payable - associated companies | 566.3 | 495.3
Long-term debt, including current portion | 1,874.0 | 1,736.9
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 1.9 | 1.5
Notes payable - associated companies | 130.1 | 48.4
Long-term debt, including current portion | 675.9 | 711.8
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 5.7 | 4.5
Notes payable | 0.0 | 0.0
Long-term debt, including current portion | 0.0 | 0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes payable - associated companies | 0.0 | 0.0
Long-term debt, including current portion | 0.0 | 0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 1.9 | 1.5
Notes payable - associated companies | 0.0 | 0.0
Long-term debt, including current portion | 0.0 | 0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 0.0 | 0.0
Notes payable | 1,317.0 | 947.0
Long-term debt, including current portion | 3,691.5 | 3,600.3
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes payable - associated companies | 566.3 | 495.3
Long-term debt, including current portion | 1,874.0 | 1,736.9
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 0.0 | 0.0
Notes payable - associated companies | 130.1 | 48.4
Long-term debt, including current portion | $ 675.9 | $ 711.8

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-10 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Name Accounting Standards Codification -Section 55 -Paragraph 10 -SubTopic 10 -Topic 825 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482881/825-10-55-10 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 28 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-28 Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-10 Reference 9: http://www.xbrl.org/2003/role/exampleRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482881/825-10-55-12 Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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