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Note 13 - Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Pension Plan [Member] |  |  |
Service cost – benefits earned during the period | $ 16.3 | $ 20.0 | $ 21.7
Interest cost on projected benefit obligation | 25.1 | 21.5 | 20.7
Expected return on plan assets | (24.5) | (31.3) | (31.6)
Amortization of prior service credit | (4.5) | (4.5) | (3.1)
Amortization of actuarial loss | 6.6 | 11.9 | 14.9
Loss on lump-sum settlements and curtailments | 9.7 | 33.6 | 18.2
Subtotal | 28.7 | 51.2 | 40.8
Regulatory adjustment | 26.3 | 12.5 | 20.6
Net pension cost | 55.0 | 63.7 | 61.4
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Service cost – benefits earned during the period | 11.4 | 14.4 | 15.4
Interest cost on projected benefit obligation | 17.8 | 14.7 | 14.2
Expected return on plan assets | (18.0) | (22.7) | (22.5)
Amortization of prior service credit | (1.9) | (1.9) | (0.6)
Amortization of actuarial loss | 6.1 | 9.6 | 11.0
Loss on lump-sum settlements and curtailments | 0.6 | 27.3 | 11.6
Subtotal | 16.0 | 41.4 | 29.1
Regulatory adjustment | 27.1 | 9.9 | 19.0
Net pension cost | 43.1 | 51.3 | 48.1
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Service cost – benefits earned during the period | 4.2 | 4.8 | 5.5
Interest cost on projected benefit obligation | 4.8 | 4.7 | 4.6
Expected return on plan assets | (3.5) | (5.2) | (5.8)
Amortization of prior service credit | (2.4) | (2.4) | (2.3)
Amortization of actuarial loss | 0.9 | 2.4 | 3.9
Loss on lump-sum settlements and curtailments | 9.1 | 6.3 | 6.6
Subtotal | 13.1 | 10.6 | 12.5
Regulatory adjustment | (1.7) | 1.7 | 0.7
Net pension cost | 11.4 | 12.3 | 13.2
Other Postretirement Benefits Plan [Member] |  |  |
Service cost – benefits earned during the period | 4.8 | 7.5 | 7.3
Interest cost on projected benefit obligation | 8.5 | 6.0 | 6.0
Expected return on plan assets | (15.8) | (16.8) | (16.1)
Amortization of prior service credit | 0.3 | 1.0 | 1.0
Amortization of actuarial loss | (4.0) | (2.2) | (1.6)
Subtotal | (6.2) | (4.5) | (3.4)
Regulatory adjustment | 0.6 | 3.3 | 13.2
Net pension cost | (5.6) | (1.2) | 9.8
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Service cost – benefits earned during the period | 4.0 | 6.3 | 6.2
Interest cost on projected benefit obligation | 6.5 | 4.5 | 4.5
Expected return on plan assets | (10.5) | (11.3) | (10.9)
Amortization of prior service credit | 0.6 | 0.7 | 0.7
Amortization of actuarial loss | (3.2) | (1.9) | (1.5)
Subtotal | (2.6) | (1.7) | (1.0)
Regulatory adjustment | 1.8 | 5.0 | 15.0
Net pension cost | (0.8) | 3.3 | 14.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Service cost – benefits earned during the period | 0.7 | 1.1 | 0.9
Interest cost on projected benefit obligation | 1.8 | 1.4 | 1.3
Expected return on plan assets | (5.0) | (5.2) | (4.9)
Amortization of prior service credit | (0.3) | 0.3 | 0.3
Amortization of actuarial loss | (0.5) | 0.0 | 0.0
Subtotal | (3.3) | (2.4) | (2.4)
Regulatory adjustment | (1.3) | (1.8) | (1.8)
Net pension cost | $ (4.6) | $ (4.2) | $ (4.2)

X
- Definition The total amount of periodic benefit cost for defined benefit plans for the period after the impact of regulatory adjustment. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments. + References No definition available. + Details
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- Definition The difference between the period's net benefit cost recognized in other comprehensive income for defined benefit plans calculated in accordance with GAAP and the amount of recovery allowed in rates by the Missouri Public Service Commission. These differences are deferred as regulatory assets or regulatory liabilities. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit 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 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit 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 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit 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 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit 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 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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