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Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Spire Missouri [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Special termination benefits |  | $ 6.1 |
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
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
Expected return on plan assets | (25.9) | (24.8) | (24.5)
Amortization of prior service credit | (4.5) | (4.5) | (4.5)
Amortization of actuarial loss | 6.5 | 6.4 | 6.6
Loss on lump-sum settlements and curtailments | 0.4 | 12.9 | 9.7
Subtotal | 17.6 | 35.0 | 28.7
Regulatory adjustment | 36.3 | 18.4 | 26.3
Net pension cost | 53.9 | 53.4 | 55.0
Special termination benefits | 0.0 | 2.6 | 0.0
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
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
Expected return on plan assets | (17.6) | (17.0) | (18.0)
Amortization of prior service credit | (1.9) | (1.9) | (1.9)
Amortization of actuarial loss | 4.1 | 5.9 | 6.1
Loss on lump-sum settlements and curtailments | 0.4 | 12.9 | 0.6
Subtotal | 12.2 | 31.8 | 16.0
Regulatory adjustment | 29.6 | 10.8 | 27.1
Net pension cost | 41.8 | 42.6 | 43.1
Special termination benefits | 0.0 | 2.6 | 0.0
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
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
Expected return on plan assets | (5.3) | (4.8) | (3.5)
Amortization of prior service credit | (2.4) | (2.4) | (2.4)
Amortization of actuarial loss | 2.4 | 1.0 | 0.9
Loss on lump-sum settlements and curtailments | 0.0 | 0.0 | 9.1
Subtotal | 5.3 | 3.5 | 13.1
Regulatory adjustment | 5.8 | 6.7 | (1.7)
Net pension cost | 11.1 | 10.2 | 11.4
Special termination benefits | 0.0 | 0.0 | 0.0
Other Postretirement Benefits Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
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
Expected return on plan assets | (16.9) | (16.0) | (15.8)
Amortization of prior service credit | 0.2 | 0.3 | 0.3
Amortization of actuarial loss | (5.8) | (5.8) | (4.0)
Subtotal | (11.0) | (2.2) | (6.2)
Regulatory adjustment | 4.5 | (3.6) | 0.6
Net pension cost | (6.5) | (5.8) | (5.6)
Special termination benefits | 0.0 | 6.1 | 0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
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
Expected return on plan assets | (11.1) | (10.6) | (10.5)
Amortization of prior service credit | 0.3 | 0.6 | 0.6
Amortization of actuarial loss | (5.1) | (5.2) | (3.2)
Subtotal | (7.2) | 1.0 | (2.6)
Regulatory adjustment | 6.2 | (1.9) | 1.8
Net pension cost | (1.0) | (0.9) | (0.8)
Special termination benefits | 0.0 | 6.1 | 0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
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
Expected return on plan assets | (5.4) | (5.1) | (5.0)
Amortization of prior service credit | (0.1) | (0.3) | (0.3)
Amortization of actuarial loss | (0.1) | (0.1) | (0.5)
Subtotal | (2.9) | (2.7) | (3.3)
Regulatory adjustment | (1.8) | (1.8) | (1.3)
Net pension cost | $ (4.7) | (4.5) | (4.6)
Special termination benefits |  | $ 0.0 | $ 0.0

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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