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Regulatory Matters - Additional Information (Details) - USD ($) $ in Thousands |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended |  |
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Oct. 24, 2025 | Oct. 23, 2025 | Sep. 12, 2025 | May 14, 2025 | Apr. 17, 2025 | Jan. 01, 2025 | Dec. 04, 2024 | Dec. 01, 2024 | Oct. 25, 2024 | Oct. 24, 2024 | Jan. 01, 2024 | Dec. 13, 2023 | Dec. 01, 2023 | Apr. 30, 2023 | Jan. 01, 2023 | Apr. 22, 2022 | Oct. 01, 2018 | Feb. 28, 2025 | Sep. 30, 2022 | Nov. 30, 2019 | Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022 | Dec. 01, 2025 | Nov. 15, 2024
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Utilities operating expense, gas and petroleum purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 905,500 | $ 1,103,300 | $ 1,260,800 |  |  |
Spire Missouri [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities approved percentage of excess natural gas supply and capacity to customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 75.00% |  |  |  |  |
Public utilities approved percentage of excess natural gas supply and capacity to retain |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |
Public utilities, value from capacity release, threshold |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200 |  |  |  |  |
Regulatory asset, percentage change of tariff |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30.00%
Utilities operating expense, gas and petroleum purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 669,400 | 886,200 | 943,400 |  |  |
Public utilities, request rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 137,400 |  |  |  |  |  |  |  |
Public utility rate base amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,379,600 |  |  |  |  |
Public utility after-tax total rate of return, infrastructure system replacement surcharge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.05% |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  |  | $ 19,000 |  |  |  |  | $ 210,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate, increase (decrease) annualized revenue |  |  |  | $ 72,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spire Missouri [Member] | Infrastructure System Replacement Surcharge [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 72,600 |  |  |  |  |  |  |  |
Spire Alabama Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Utilities operating expense, gas and petroleum purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 154,400 | 189,500 | 202,700 |  |  |
Public utilities, rate increase decrease index range margin |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.50% |  |  |  |  | 1.50% |  |
Financing Receivable, after Allowance for Credit Loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,400 | 5,900 |  |  |  |
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities approved percentage of excess natural gas supply and capacity to customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 75.00% |  |  |  |  |  |
Public utilities approved percentage of excess natural gas supply and capacity to retain |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |  |
Public utilities, value from capacity release, threshold |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,600 |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,000 | $ 2,500 | $ 4,400 |  |  |
Public utilities maximum return on equity percentage of prior year revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |  |  |  |  | 4.00% |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.40% |  | 9.70% |  |  |  |  |  |  |
Public utilities, rate increase decrease index range margin |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.10% |  |  |  |  | 0.10% |  |
Public utilities, equity limitation as percent of total capitalization |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55.50% |  |  |  |  | 55.50% |  |
Public utilities, equity limitation as percent of average common equity growth |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  |  | 6.00% |  |
Annual revenue increase from rates, amount |  |  |  |  |  |  |  | $ 4,000 |  | $ 3,600 | $ 14,300 |  |  |  | $ 15,000 |  |  |  |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 17,200 |  |
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.15% |  | 9.50% |  |  |  |  |  |  |
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.65% |  | 9.90% |  |  |  |  |  |  |
Spire Alabama Inc [Member] | Subsequent Event [Member] | Alabama Public Service Commission [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, equity limitation as percent of average common equity growth |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.24% |
Annual revenue increase from rates, amount | $ 15,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spire Gulf [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.95% |  |  |  |  |
Public utilities, rate increase decrease index range margin |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.50% |  |  |  |  | 1.50% |  |
Public utilities approved rate reflected benefit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200 |  |  |  |  |
Annual revenue increase from rates, amount |  |  |  |  |  |  |  |  | $ 1,900 |  |  | $ 2,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate, increase (decrease) annualized revenue |  |  |  |  |  |  | $ 1,300 |  |  |  |  |  | $ 2,000 | $ 1,800 |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Return on Equity, Pro-Rata, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 75.00% |  |  |  |  |
Regulated Operating Revenue Variances Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100 |  |  |  |  |
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Single Force Majeure Event Deferred Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100 |  |  |  |  |
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Multiple Force Majeure Events Deferred Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 150 |  |  |  |  |
Regulated Operations, Recovery of Incurred Costs, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90.00% |  |  |  |  |
Spire Gulf [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.70% |  |  |  |  |
Spire Gulf [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.30% |  |  |  |  |
Spire Gulf [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual revenue increase from rates, amount |  | $ 3,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spire [Member] | Maximum [Member] | Related Party [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, after Allowance for Credit Loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,000 |  |  |  |  |
Spire [Member] | Alabama Public Service Commission [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 75,000 |  |  |  |  |
Spire Mississippi [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  | $ 800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.73% |  |  |  |  |
Public utilities, approved rate, increase (decrease) annualized revenue |  |  |  |  |  | $ 600 |  |  |  |  | $ 1,000 |  |  |  | $ 800 |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Return on Equity, Excess of Allowed Return, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |
Public Utilities, Approved Return on Equity Shortfall Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 75.00% |  |  |  |  |
Public Utilities, Approved Return On Equity, Excess, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |
Future Income Taxes Due From Customers and Pension and Other Postretirement Benefit Costs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20 years |  |  |  |  |
PGA Assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |
PGA Assets Not Earning a Return [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |
Related Debt Term and Diaphragm [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20 years |  |  |  |  |

X
- Definition The amount of annual revenue increase from new rates. + References No definition available. + Details
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- Definition Approved percentage of excess natural gas supply and capacity to customers under public utilities. + References No definition available. + Details
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- Definition Approved percentage of excess natural gas supply and capacity to retain for public utilities. + References No definition available. + Details
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- Definition Public Utilities, Approved Rate Increase (Decrease), Annualized Revenue + References No definition available. + Details
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- Definition Public utilities approved rate net income benefit. + References No definition available. + Details
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- Definition Percentage of excess of allowed return of approved return on equity of public utilities. + References No definition available. + Details
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- Definition Percentage of excess of approved return on equity for public utilities. + References No definition available. + Details
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- Definition Pro-rata percentage of approved return on equity under public utilities. + References No definition available. + Details
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- Definition Percentage of shortfall for approved return on equity of public utilities. + References No definition available. + Details
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- Definition Represents the equity limitation as a percentage of average common equity growth. + References No definition available. + Details
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- Definition Represents equity limitation as a percentage of total capitalization by regulatory agency. + References No definition available. + Details
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- Definition Percentage of prior year revenues for maximum return on equity under public utilities. + References No definition available. + Details
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- Definition Represents rate increase decrease index range margin for public utilities. + References No definition available. + Details
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- Definition Represents the threshold value from capacity release with regulatory agency. + References No definition available. + Details
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- Definition Public utility after-tax total rate of return, infrastructure system replacement surcharge. + References No definition available. + Details
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- Definition Public utility rate base amount. + References No definition available. + Details
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- Definition Variance amount of revenue for regulated operating. + References No definition available. + Details
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- Definition Percentage of recovery of incurred costs under regulated operations. + References No definition available. + Details
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- Definition Regulatory asset percentage change of tariff + References No definition available. + Details
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