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Income Taxes - Significant Items Comprising the Net Deferred Tax Liability or Asset (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
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Spire Missouri [Member] |  |  |
Deferred tax assets: |  |  |
Operating losses |  | $ 1.5 | $ 24.7
Goodwill |  | 0.0 | 0.0
Pension and other postretirement benefits |  | 36.0 | 39.3
Regulatory amount due to customers, net |  | 21.4 | 24.5
Reserves not currently deductible |  | 0.0 | 0.0
Other |  | 16.7 | 12.3
Total deferred tax assets |  | 75.6 | 100.8
Deferred tax liabilities: |  |  |
Relating to property |  | (572.6) | (531.2)
Regulatory pension and other postretirement benefits |  | (27.2) | (33.8)
Deferred gas costs |  | (8.2) | (17.7)
Other | [1] | (66.3) | (85.7)
Total deferred tax liabilities |  | (674.3) | (668.4)
Net deferred tax (liability) asset |  | (598.7) | (567.6)
Spire Alabama Inc [Member] |  |  |
Deferred tax assets: |  |  |
Operating losses |  | 146.1 | 153.0
Goodwill |  | 29.7 | 44.4
Pension and other postretirement benefits |  | 14.8 | 17.1
Regulatory amount due to customers, net |  | 0.0 | 2.3
Reserves not currently deductible |  | 3.4 | 2.9
Other |  | 6.0 | 2.1
Total deferred tax assets |  | 200.0 | 221.8
Deferred tax liabilities: |  |  |
Relating to property |  | (238.1) | (230.0)
Regulatory pension and other postretirement benefits |  | (24.0) | (25.6)
Deferred gas costs |  | 0.0 | 0.0
Other | [1] | (2.0) | (2.1)
Total deferred tax liabilities |  | (264.1) | (257.7)
Net deferred tax (liability) asset |  | (64.1) | (35.9)
Parent Company [Member] |  |  |
Deferred tax assets: |  |  |
Operating losses |  | 150.1 | 182.2
Goodwill |  | 0.0 | 0.0
Pension and other postretirement benefits |  | 52.6 | 58.6
Regulatory amount due to customers, net |  | 23.9 | 27.1
Reserves not currently deductible |  | 24.9 | 0.5
Other |  | 73.5 | 43.3
Total deferred tax assets |  | 325.0 | 311.7
Deferred tax liabilities: |  |  |
Relating to property |  | (912.3) | (842.2)
Regulatory pension and other postretirement benefits |  | (54.4) | (60.8)
Deferred gas costs |  | (10.7) | (19.9)
Other | [1] | (235.0) | (197.2)
Total deferred tax liabilities |  | (1,212.4) | (1,120.1)
Net deferred tax (liability) asset |  | $ (887.4) | $ (808.4)
[1] | For Spire, Other consists primarily of goodwill-related liabilities.

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