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Pension Plans and Other Postretirement Benefits - Reconciliation of Pension Benefit Obligation (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Spire Missouri [Member] |  |  |
Special termination benefits |  | $ 6.1 |
Pension Plan [Member] |  |  |
Benefit obligation, beginning of year | $ 469.7 | 438.7 |
Service cost – benefits earned during the period | 17.6 | 15.7 | $ 16.3
Interest cost on projected benefit obligation | 23.5 | 26.7 | 25.1
Actuarial loss (gain) | (15.3) | 46.0 |
Plan amendments | 0.0 | 0.2 |
Special termination benefits | 0.0 | 2.6 |
Settlement benefits paid | (1.5) | (37.3) |
Regular benefits paid | (43.8) | (22.9) |
Benefit obligation, end of year | 450.2 | 469.7 | 438.7
Accumulated benefit obligation, end of year | 439.6 | 459.4 |
Special termination benefits | 0.0 | 2.6 | 0.0
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Benefit obligation, beginning of year | 319.7 | 313.2 |
Service cost – benefits earned during the period | 11.4 | 10.4 | 11.4
Interest cost on projected benefit obligation | 15.8 | 18.9 | 17.8
Actuarial loss (gain) | (8.5) | 25.2 |
Plan amendments | 0.0 | 0.2 |
Special termination benefits | 0.0 | 2.6 |
Settlement benefits paid | (1.5) | (37.3) |
Regular benefits paid | (29.7) | (13.5) |
Benefit obligation, end of year | 307.2 | 319.7 | 313.2
Accumulated benefit obligation, end of year | 302.8 | 314.7 |
Special termination benefits | 0.0 | 2.6 | 0.0
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Benefit obligation, beginning of year | 97.9 | 80.6 |
Service cost – benefits earned during the period | 5.5 | 4.6 | 4.2
Interest cost on projected benefit obligation | 5.1 | 5.1 | 4.8
Actuarial loss (gain) | (4.3) | 14.3 |
Plan amendments | 0.0 | 0.0 |
Special termination benefits | 0.0 | 0.0 |
Settlement benefits paid | 0.0 | 0.0 |
Regular benefits paid | (11.2) | (6.7) |
Benefit obligation, end of year | 93.0 | 97.9 | 80.6
Accumulated benefit obligation, end of year | 87.3 | 92.9 |
Special termination benefits | 0.0 | 0.0 | 0.0
Other Postretirement Benefits Plan [Member] |  |  |
Benefit obligation, beginning of year | 142.5 | 142.6 |
Service cost – benefits earned during the period | 4.2 | 4.3 | 4.8
Interest cost on projected benefit obligation | 7.3 | 8.9 | 8.5
Actuarial loss (gain) | (18.4) | (1.5) |
Plan amendments | 0.0 | (4.7) |
Regular benefits paid | (4.1) | (13.4) |
Benefit obligation, end of year | 131.5 | 142.5 | 142.6
Retiree drug subsidy program | 0.0 | 0.2 |
Special termination benefits | 0.0 | 6.1 | 0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Benefit obligation, beginning of year | 102.4 | 105.3 |
Service cost – benefits earned during the period | 3.5 | 3.5 | 4.0
Interest cost on projected benefit obligation | 5.2 | 6.6 | 6.5
Actuarial loss (gain) | (17.1) | (3.2) |
Plan amendments | 0.0 | (4.7) |
Regular benefits paid | (1.9) | (11.4) |
Benefit obligation, end of year | 92.1 | 102.4 | 105.3
Retiree drug subsidy program | 0.0 | 0.2 |
Special termination benefits | 0.0 | 6.1 | 0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Benefit obligation, beginning of year | 39.0 | 35.1 |
Service cost – benefits earned during the period | 0.7 | 0.7 | 0.7
Interest cost on projected benefit obligation | 2.0 | 2.1 | 1.8
Actuarial loss (gain) | (1.2) | 3.1 |
Regular benefits paid | (2.2) | (2.0) |
Benefit obligation, end of year | $ 38.3 | 39.0 | 35.1
Special termination benefits |  | $ 0.0 | $ 0.0

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- 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 5 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: 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 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 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 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 Reference 3: 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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