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Income Taxes - Effective Income Tax Rate (Details) | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Spire Missouri [Member] |  |  |  |
Effective Income Tax Rate Reconciliation [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 21.00%
State and local income taxes, net of federal income tax benefits |  | 2.60% | 2.60% | 2.60%
Certain expenses capitalized on books and deducted on tax return |  | (2.50%) | (2.70%) | (2.70%)
Tax credits | [1] | (1.10%) | (1.10%) | (4.20%)
Amortization of excess deferred taxes |  | (7.00%) | (7.40%) | (7.70%)
Taxes related to prior years |  | (2.90%) | 0.00% | 0.00%
Other items - net |  | (0.40%) | (0.70%) | 0.00%
Effective income tax rate |  | 9.70% | 11.70% | 9.00%
Spire Alabama Inc [Member] |  |  |  |
Effective Income Tax Rate Reconciliation [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 21.00%
State and local income taxes, net of federal income tax benefits |  | 4.10% | 4.10% | 4.10%
Certain expenses capitalized on books and deducted on tax return |  | 0.00% | 0.00% | 0.00%
Tax credits | [1] | (0.30%) | (0.30%) | (1.90%)
Amortization of excess deferred taxes |  | 0.10% | 0.20% | 0.20%
Taxes related to prior years |  | 0.00% | 0.00% | 0.00%
Other items - net |  | 0.10% | 0.10% | 0.10%
Effective income tax rate |  | 25.00% | 25.10% | 23.50%
Parent Company [Member] |  |  |  |
Effective Income Tax Rate Reconciliation [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 21.00%
State and local income taxes, net of federal income tax benefits |  | 3.70% | 3.70% | 3.70%
Certain expenses capitalized on books and deducted on tax return |  | (1.10%) | (1.20%) | (1.40%)
Tax credits | [1] | (0.80%) | (0.90%) | (3.30%)
Amortization of excess deferred taxes |  | (3.10%) | (3.30%) | (5.10%)
Taxes related to prior years |  | (1.30%) | 0.00% | 0.00%
Other items - net |  | (0.40%) | (0.30%) | 0.20%
Effective income tax rate |  | 18.00% | 19.00% | 15.10%
[1] | In 2023, the Company completed a research and development study which encompassed fiscal years 2014 to 2022.

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- Definition Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to amortization of excess deferred taxes. + References No definition available. + Details
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