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Note 13 - Pension Plans and Other Postretirement Benefits - Targeted and Actual Plan Assets by Category (Details) | Sep. 30, 2023 | Sep. 30, 2022
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Pension Plan [Member] | Spire Missouri [Member] |  |
Return seeking assets, target | 100.00% | 100.00%
Return seeking assets, actual | 100.00% | 100.00%
Pension Plan [Member] | Spire Missouri [Member] | Return Seeking Assets [Member] |  |
Return seeking assets, target | 70.00% | 70.00%
Return seeking assets, actual | 77.00% | 74.30%
Pension Plan [Member] | Spire Missouri [Member] | Liability Hedging Assets [Member] |  |
Return seeking assets, target | 30.00% | 30.00%
Return seeking assets, actual | 20.00% | 22.80%
Pension Plan [Member] | Spire Missouri [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] |  |
Return seeking assets, target | 0.00% | 0.00%
Return seeking assets, actual | 3.00% | 2.90%
Pension Plan [Member] | Spire Alabama Inc [Member] |  |
Return seeking assets, target | 100.00% | 100.00%
Return seeking assets, actual | 100.00% | 100.00%
Pension Plan [Member] | Spire Alabama Inc [Member] | Return Seeking Assets [Member] |  |
Return seeking assets, target | 75.00% | 70.00%
Return seeking assets, actual | 76.00% | 68.70%
Pension Plan [Member] | Spire Alabama Inc [Member] | Liability Hedging Assets [Member] |  |
Return seeking assets, target | 25.00% | 30.00%
Return seeking assets, actual | 15.00% | 25.30%
Pension Plan [Member] | Spire Alabama Inc [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] |  |
Return seeking assets, target | 0.00% | 0.00%
Return seeking assets, actual | 9.00% | 6.00%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |
Return seeking assets, target | 100.00% |
Return seeking assets, actual | 100.00% | 100.00%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Defined Benefit Plan, Equity Securities [Member] |  |
Return seeking assets, target | 60.00% |
Return seeking assets, actual | 58.90% | 58.70%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Debt Securities [Member] |  |
Return seeking assets, target | 40.00% |
Return seeking assets, actual | 38.10% | 40.50%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] |  |
Return seeking assets, target | 0.00% |
Return seeking assets, actual | 3.00% | 0.80%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |
Return seeking assets, target | 100.00% |
Return seeking assets, actual | 100.00% | 100.00%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] | Defined Benefit Plan, Equity Securities [Member] |  |
Return seeking assets, target | 60.50% |
Return seeking assets, actual | 59.90% | 57.70%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] | Debt Securities [Member] |  |
Return seeking assets, target | 39.50% |
Return seeking assets, actual | 40.10% | 42.30%

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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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