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Pension Plans and Other Postretirement Benefits - Additional Information (Details) |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 26, 2022 USD ($) | Dec. 25, 2022 USD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Dec. 26, 2021 USD ($) | Dec. 25, 2021 USD ($) | Dec. 23, 2021 USD ($)
Regulatory Asset, Total |  |  | $ 1,401,800,000 | $ 1,367,200,000 |  |  |  |
Defined benefit plan, benefit obligation, actuarial losses prior years, percent |  |  | 0.0040 |  |  |  |  |
Defined benefit plan, benefit obligation, actuarial gains from prior years, percent |  |  | 0.45% |  |  |  |  |
Implementation Period Used To Calculate Return On Plan Assets (Year) |  |  | 4 years |  |  |  |  |
Gains Or Losses Includible In Cost Amortized To Extent Exceeds Projected Benefit Obligations or Market Related Value of Plan Assets Percentage |  |  | 10.00% |  |  |  |  |
Regulatory Liability |  |  | $ 617,400,000 | 585,000,000 |  |  |  |
Defined Contribution Plan, Cost |  |  | 16,500,000 | 16,600,000 | $ 15,500,000 |  |  |
Pension Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | 75,800,000 | 38,500,000 |  |  |  |
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  | 0 | 2,600,000 | 0 |  |  |
Other Postretirement Benefits Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | 2,700,000 | 200,000 |  |  |  |
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  | $ 0 | $ 6,100,000 | $ 0 |  |  |
Spire Missouri [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate |  |  | 5.55% | 5.10% | 6.25% |  |  |
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement |  |  | $ 1,500,000 | $ 37,300,000 |  |  |  |
Regulatory Asset, Total |  |  | 702,100,000 | 672,000,000 |  |  |  |
Regulatory Liability |  |  | 495,200,000 | 453,500,000 |  |  |  |
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  |  | 6,100,000 |  |  |  |
Defined Contribution Plan, Cost |  |  | 9,300,000 | 9,300,000 | $ 9,100,000 |  |  |
Spire Missouri [Member] | Pension Costs [Member] | Missouri West [Member] |  |  |  |  |  |  |  |
Regulatory Asset, Total |  |  |  |  |  | $ 4,400,000 | $ 5,500,000 |
Spire Missouri [Member] | Pension Costs [Member] | Missouri East [Member] |  |  |  |  |  |  |  |
Regulatory Asset, Total |  |  |  |  |  | $ 29,900,000 | $ 32,400,000 |
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri West [Member] |  |  |  |  |  |  |  |
Regulatory Liabilities For Which No Return On Investment During Recovery Period Is Provided | $ 800,000 | $ 1,100,000 |  |  |  |  |  |
Remaining Recovery Period of Regulatory Liabilities for which No Return on Investment During Recovery Period is Provided (Year) | 8 years | 8 years |  |  |  |  |  |
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri East [Member] |  |  |  |  |  |  |  |
Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided | $ 6,900,000 | $ 11,000,000 |  |  |  |  |  |
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year) |  | 8 years |  |  |  |  |  |
Additional Remaining Recovery Period of Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided (Year) | 8 years |  |  |  |  |  |  |
Spire Missouri [Member] | Pension Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | 37,200,000 | 25,400,000 |  |  |  |
Anticipated Contributions to Pension Plans Qualified Trusts |  |  | 22,100,000 |  |  |  |  |
Anticipated Contributions To Pension Plans Non Qualified Trusts |  |  | 600,000 |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  | 0 | 2,600,000 | 0 |  |  |
Spire Missouri [Member] | Pension Plan [Member] | Nonqualified Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | 0 |  |  |  |  |
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | 2,700,000 | 200,000 |  |  |  |
Postretirement Benefit Plans, Recovery in Rates Base Allowance |  |  |  |  |  |  |  | $ 8,600,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  | 0 | 6,100,000 | 0 |  |  |
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Missouri West [Member] |  |  |  |  |  |  |  |
Regulatory Liability | $ 100,000 |  |  |  |  |  |  |
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Missouri East [Member] |  |  |  |  |  |  |  |
Regulatory Liability | $ 900,000 |  |  |  |  |  |  |
Spire Alabama Inc [Member] |  |  |  |  |  |  |  |
Regulatory Asset, Total |  |  | 666,700,000 | 661,200,000 |  |  |  |
Regulatory Liability |  |  | 49,500,000 | 62,100,000 |  |  |  |
Defined Contribution Plan, Cost |  |  | $ 3,000,000 | $ 2,900,000 | 3,100,000 |  |  |
Spire Alabama Inc [Member] | Pension Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate |  |  | 5.55% | 5.10% |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | $ 30,100,000 | $ 12,600,000 |  |  |  |
Anticipated Contributions to Pension Plans Qualified Trusts |  |  | 7,000,000 |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  | $ 0 | 0 | 0 |  |  |
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |  |  |  |  |
Anticipated Contributions to Pension Plans Qualified Trusts |  |  |  | 0 |  |  |  |
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits |  |  |  | $ 0 | $ 0 |  |  |
Spire Alabama And Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate |  |  | 5.50% | 5.05% |  |  |  |

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