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Note 13 - Pension Plans and Other Postretirement Benefits - Reconciliation of Pension Benefit Obligation (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Settlement benefits paid | $ 0 | $ 0 |
Pension Plan [Member] |  |  |
Benefit obligation, beginning of year | 464,400 | 689,600 |
Service cost – benefits earned during the period | 16,300 | 20,000 | $ 21,700
Interest cost on projected benefit obligation | 25,100 | 21,500 | 20,700
Actuarial (gain) loss | (1,100) | (173,900) |
Settlement loss | 9,700 | 33,400 |
Settlement benefits paid | (48,800) | (109,300) |
Regular benefits paid | (26,900) | (16,900) |
Benefit obligation, end of year | 438,700 | 464,400 | 689,600
Accumulated benefit obligation, end of year | 433,100 | 457,100 |
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Benefit obligation, beginning of year | 320,600 | 479,000 |
Service cost – benefits earned during the period | 11,400 | 14,400 | 15,400
Interest cost on projected benefit obligation | 17,800 | 14,700 | 14,200
Actuarial (gain) loss | (4,900) | (116,200) |
Settlement loss | 600 | 27,900 |
Settlement benefits paid | (9,900) | (87,000) |
Regular benefits paid | (22,400) | (12,200) |
Benefit obligation, end of year | 313,200 | 320,600 | 479,000
Accumulated benefit obligation, end of year | 309,500 | 315,000 |
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Benefit obligation, beginning of year | 97,800 | 149,400 |
Service cost – benefits earned during the period | 4,200 | 4,800 | 5,500
Interest cost on projected benefit obligation | 4,800 | 4,700 | 4,600
Actuarial (gain) loss | 5,400 | (42,400) |
Settlement loss | 9,100 | 5,600 |
Settlement benefits paid | (38,800) | (22,300) |
Regular benefits paid | (1,900) | (2,000) |
Benefit obligation, end of year | 80,600 | 97,800 | 149,400
Accumulated benefit obligation, end of year | 78,800 | 96,200 |
Other Postretirement Benefits Plan [Member] |  |  |
Benefit obligation, beginning of year | 148,100 | 204,200 |
Service cost – benefits earned during the period | 4,800 | 7,500 | 7,300
Interest cost on projected benefit obligation | 8,500 | 6,000 | 6,000
Actuarial (gain) loss | (10,600) | (52,600) |
Regular benefits paid | (8,400) | (11,000) |
Benefit obligation, end of year | 142,600 | 148,100 | 204,200
Plan amendments | 0 | (6,200) |
Retiree drug subsidy program | 200 | 200 |
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Benefit obligation, beginning of year | 114,800 | 151,700 |
Service cost – benefits earned during the period | 4,000 | 6,300 | 6,200
Interest cost on projected benefit obligation | 6,500 | 4,500 | 4,500
Actuarial (gain) loss | (13,400) | (37,700) |
Regular benefits paid | (6,800) | (9,200) |
Benefit obligation, end of year | 105,300 | 114,800 | 151,700
Plan amendments | 0 | (1,000) |
Retiree drug subsidy program | 200 | 200 |
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Benefit obligation, beginning of year | 29,900 | 48,000 |
Service cost – benefits earned during the period | 700 | 1,100 | 900
Interest cost on projected benefit obligation | 1,800 | 1,400 | 1,300
Actuarial (gain) loss | 4,200 | (13,600) |
Regular benefits paid | (1,500) | (1,800) |
Benefit obligation, end of year | 35,100 | 29,900 | $ 48,000
Plan amendments | 0 | (5,200) |
Retiree drug subsidy program | $ 0 | $ 0 |

X
- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of actuarial present value of benefits attributed to service rendered by employee for 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 (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 60 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480266/715-60-50-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(6) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of (increase) decrease to benefit obligation of defined benefit plan from remeasurement due to settlement. Excludes decrease to benefit obligation for settlement payment. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(10) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(10) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 60 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480266/715-60-50-4 + Details
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