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Note 12 - Income Taxes - Effective Income Tax Rate (Details) | 12 Months Ended
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Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2020
Spire Missouri [Member] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 21.00%
State and local income taxes, net of federal income tax benefits |  | 2.60% | 2.60% | 2.60%
Certain expenses capitalized on books and deducted on tax return |  | (3.20%) | (3.30%) | (4.60%)
Taxes related to prior years |  | (0.60%) | (0.20%) | (1.40%)
Amortization of excess deferred taxes |  | (7.30%) | (5.00%) | (5.70%)
Other items – net * | [1] | 3.10% | (0.20%) | (0.20%)
Effective income tax rate |  | 15.60% | 14.90% | 11.70%
Spire Alabama Inc [Member] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 21.00%
State and local income taxes, net of federal income tax benefits |  | 4.10% | 4.10% | 4.10%
Certain expenses capitalized on books and deducted on tax return |  | 0.00% | 0.00% | 0.00%
Taxes related to prior years |  | 0.00% | 0.00% | 0.10%
Amortization of excess deferred taxes |  | 0.00% | 0.00% | 0.00%
Other items – net * | [1] | 0.20% | 0.20% | (0.10%)
Effective income tax rate |  | 25.30% | 25.30% | 25.10%
Parent Company [Member] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 21.00%
State and local income taxes, net of federal income tax benefits |  | 3.60% | 3.60% | 9.00%
Certain expenses capitalized on books and deducted on tax return |  | (1.60%) | (1.60%) | (6.60%)
Taxes related to prior years |  | (0.30%) | (0.50%) | (1.80%)
Amortization of excess deferred taxes |  | (3.50%) | (2.50%) | (8.30%)
Other items – net * | [1] | 1.90% | 0.10% | (1.00%)
Effective income tax rate |  | 21.10% | 20.10% | 12.30%
[1] | Other consists primarily of property adjustments.

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- Definition Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to amortization of excess deferred taxes. + References No definition available. + Details
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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-12 + Details
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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(h)(2)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-12 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 6.I.Fact.4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1 + Details
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- Definition Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to depreciation and amortization. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-12 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 6.I.Fact.4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1 + Details
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- Definition Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-12 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 6.I.Fact.4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1 + Details
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- Definition Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to revisions of previously reported income tax expense. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-12 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 6.I.Fact.4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1 + Details
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