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Pension Plans and Other Postretirement Benefits - Other Changes in Plan Assets and Pension Benefit Obligations (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Current year actuarial (gain) loss | $ (11.9) | $ 8.8 | $ 14.5
Amortization of actuarial loss | (6.5) | (6.4) | (6.6)
Acceleration of loss recognized due to settlement | (0.4) | (12.9) | (9.7)
Current year service credit | 0.0 | 0.2 | 0.0
Amortization of prior service credit | 4.5 | 4.5 | 4.5
Subtotal | (14.3) | (5.8) | 2.7
Regulatory adjustment | 13.0 | 5.2 | (2.8)
Total recognized in OCI | (1.3) | (0.6) | (0.1)
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Current year actuarial (gain) loss | (9.9) | (0.9) | 5.7
Amortization of actuarial loss | (4.1) | (5.9) | (6.1)
Acceleration of loss recognized due to settlement | (0.4) | (12.9) | (0.6)
Current year service credit | 0.0 | 0.2 | 0.0
Amortization of prior service credit | 1.9 | 1.9 | 1.9
Subtotal | (12.5) | (17.6) | 0.9
Regulatory adjustment | 11.2 | 17.0 | (1.0)
Total recognized in OCI | (1.3) | (0.6) | (0.1)
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Current year actuarial (gain) loss | (0.8) | 8.5 | 9.0
Amortization of actuarial loss | (2.4) | (1.0) | (0.9)
Acceleration of loss recognized due to settlement | 0.0 | 0.0 | (9.1)
Current year service credit | 0.0 | 0.0 | 0.0
Amortization of prior service credit | 2.4 | 2.4 | 2.4
Subtotal | (0.8) | 9.9 | 1.4
Regulatory adjustment | 0.8 | (9.9) | (1.4)
Total recognized in OCI | 0.0 | 0.0 | 0.0
Other Postretirement Benefits Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Current year actuarial (gain) loss | (36.5) | (52.4) | (24.4)
Amortization of actuarial loss | 5.8 | 5.8 | 4.0
Amortization of prior service credit | (0.2) | (0.3) | (0.3)
Subtotal | (30.9) | (51.6) | (20.7)
Regulatory adjustment | 30.9 | 51.6 | 20.7
Total recognized in OCI | 0.0 | 0.0 | 0.0
Current year prior service cost | 0.0 | (4.7) | 0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Current year actuarial (gain) loss | (31.9) | (40.4) | (21.5)
Amortization of actuarial loss | 5.1 | 5.2 | 3.2
Amortization of prior service credit | (0.3) | (0.6) | (0.6)
Subtotal | (27.1) | (40.5) | (18.9)
Regulatory adjustment | 27.1 | 40.5 | 18.9
Total recognized in OCI | 0.0 | 0.0 | 0.0
Current year prior service cost |  | (4.7) | 0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Current year actuarial (gain) loss | (4.0) | (9.3) | (1.1)
Amortization of actuarial loss | 0.1 | 0.1 | 0.5
Amortization of prior service credit | 0.1 | 0.3 | 0.3
Subtotal | (3.8) | (8.9) | (0.3)
Regulatory adjustment | 3.8 | 8.9 | 0.3
Total recognized in OCI | $ 0.0 | 0.0 | 0.0
Current year prior service cost |  | $ 0.0 | $ 0.0

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