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Note 12 - Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2020
Spire Missouri [Member] |  |  |
Unrecognized tax benefits, beginning of year | $ 16.1 | $ 13.0 | $ 10.4
Increases related to tax positions taken in current year | 3.2 | 3.1 | 2.6
Reductions due to lapse of applicable statute of limitations | 0.0 | 0.0 | 0.0
Unrecognized tax benefits, end of year | 19.3 | 16.1 | 13.0
Parent Company [Member] |  |  |
Unrecognized tax benefits, beginning of year | 16.4 | 13.2 | 10.7
Increases related to tax positions taken in current year | 3.3 | 3.2 | 2.6
Reductions due to lapse of applicable statute of limitations | (0.1) | 0.0 | (0.1)
Unrecognized tax benefits, end of year | $ 19.6 | $ 16.4 | $ 13.2

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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 15A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-15A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 10B -Publisher FASB -URI https://asc.fasb.org//1943274/2147482525/740-10-45-10B + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 15A -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-15A + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 15A -Subparagraph (a)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-15A + Details
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- Details
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- Details
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