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Note 12 - Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Spire Missouri [Member] |  |  |
Unrecognized tax benefits, beginning of year | $ 19,300 | $ 16,100 | $ 13,000
Increases related to tax positions taken in current year | 3,600 | 3,200 | 3,100
Reductions due to lapse of applicable statute of limitations | 0 | 0 | 0
Unrecognized tax benefits, end of year | 22,900 | 19,300 | 16,100
Spire Alabama Inc [Member] |  |  |
Unrecognized tax benefits, beginning of year | 0 | 0 | 0
Increases related to tax positions taken in current year | 300 | 0 | 0
Reductions due to lapse of applicable statute of limitations | 0 | 0 | 0
Unrecognized tax benefits, end of year | 300 | 0 | 0
Parent Company [Member] |  |  |
Unrecognized tax benefits, beginning of year | 19,600 | 16,400 | 13,200
Increases related to tax positions taken in current year | 4,200 | 3,300 | 3,200
Reductions due to lapse of applicable statute of limitations | 0 | (100) | 0
Unrecognized tax benefits, end of year | $ 23,800 | $ 19,600 | $ 16,400

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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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