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Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Consolidated Statements of Income and Comprehensive Income (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Derivative [Line Items] |  |  |  |
Net hedging gain (loss) arising during the period |  | $ 12.8 | $ (15.9) | $ 20.1
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Other Comprehensive Income (Loss) [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Net hedging gain (loss) arising during the period |  | (9.6) | (4.1) | (16.7)
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Loss Effective Portion Net |  | 5.1 | 11.1 | 2.4
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Comprehensive Income (Loss) [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Net hedging gain (loss) arising during the period |  | 12.8 | (15.9) | 20.1
Not Designated as Hedging Instrument [Member] | Gasoline And Heating Oil Contracts [Member] | Non Regulated Gas Marketing Operating Revenue [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivatives Not Designated as Hedging Instrument |  |  | (0.3) | 0.3
Not Designated as Hedging Instrument [Member] | Natural Gas Commodity Contracts [Member] | Non Regulated Gas Marketing Operating Revenue [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivatives Not Designated as Hedging Instrument | [1] | (6.5) | 12.5 | 18.0
Not Designated as Hedging Instrument [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Non Regulated Gas Marketing Operating Revenue [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivatives Not Designated as Hedging Instrument | [1] | $ (3.1) | $ (16.3) | $ (35.0)
[1] | Gains and losses on Spire Missouri’s natural gas derivative instruments, which are not designated as hedging instruments for financial reporting purposes, are deferred pursuant to the Missouri Utilities’ PGA clauses and initially recorded as regulatory assets or regulatory liabilities. These gains and losses are excluded from the table above because they have no direct impact on the statements of income. Such amounts are recognized in the statements of income as a component of natural gas operating expenses when they are recovered through the PGA clause and reflected in customer billings.

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- Definition Derivative instruments gain loss reclassified from accumulated OCI into income loss effective portion net. + References No definition available. + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 182 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480401/815-10-55-182 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4CC -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4CC Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 21 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147476148/220-40-50-21 + Details
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- Definition Amount, before tax and reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 182 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480401/815-10-55-182 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4C -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4C Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-11 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480627/815-20-45-3 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 10A -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A + Details
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