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Note 13 - Pension Plans and Other Postretirement Benefits - Assumption Used (Details) | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Pension Plan [Member] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% |
Spire Missouri East [Member] | Pension Plan [Member] |  |  |
Weighted average discount rate | 5.70% | 3.00% | 2.85%
Weighted average discount rate | 6.25% | 5.70% |
Spire Missouri East [Member] | Other Postretirement Benefits Plan [Member] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% |
Spire Alabama Inc [Member] | Pension Plan [Member] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 6.50% | 6.75% | 6.75%
Spire Alabama Inc [Member] | Pension Plan [Member] | Maximum [Member] |  |  |
Weighted average discount rate | 5.65% | 3.10% | 2.80%
Weighted average discount rate | 6.20% | 5.70% |
Spire Alabama Inc [Member] | Pension Plan [Member] | Minimum [Member] |  |  |
Weighted average discount rate | 5.65% | 3.10% | 2.95%
Weighted average discount rate | 6.25% | 5.65% |
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member] |  |  |
Weighted average discount rate | 5.80% | 2.95% | 2.75%
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member] | Maximum [Member] |  |  |
Expected long-term rate of return on plan assets | 6.00% | 6.25% | 6.25%
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member] | Minimum [Member] |  |  |
Expected long-term rate of return on plan assets | 4.75% | 5.00% | 5.00%
Spire Missouri [Member] |  |  |
Weighted average discount rate | 6.30% | 5.80% |
Spire Missouri [Member] | Pension Plan [Member] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 6.50% | 6.75% | 6.75%
Weighted average discount rate |  |  | 3.00%
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] |  |  |
Weighted average discount rate | 5.80% | 2.95% | 2.75%
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 5.50% | 5.75% | 5.75%
Spire Alabama And Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] |  |  |
Weighted average discount rate | 6.30% | 5.80% |
Spire Missouri West [Member] | Pension Plan [Member] |  |  |
Weighted average discount rate | 5.80% | 3.00% | 2.75%
Weighted average discount rate | 6.30% | 5.80% |
Spire Alabama or Spire Missouri [Member] | Pension Plan [Member] |  |  |
Cash balance interest crediting rate - Spire Alabama / Spire Missouri | 4.25% | 4.25% |

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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted-average interest crediting rate used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, cash balance and other defined benefit plans with promised interest crediting rate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-8 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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