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Shareholders' Equity - Additional Information (Details) - USD ($) |  |  |  | 1 Months Ended | 3 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 05, 2024 | Dec. 11, 2023 | May 21, 2019 | Dec. 31, 2024 | Feb. 28, 2021 | Sep. 30, 2024 | Oct. 23, 2025 | Sep. 30, 2025 | Mar. 31, 2025 | Mar. 01, 2024 | Jan. 25, 2024 | Sep. 30, 2022
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 5,000,000 |  | 5,000,000 |  |  |  |
Number of Shares Authorized for Issuance (in shares) |  |  |  |  |  |  |  | 250,000 |  |  |  |
Number Of Shares Remaining and Available for Issuance (in shares) |  |  |  |  |  | 218,141 |  | 196,135 |  |  |  |
Retained Earnings Dividend Restrictions Formula Component Fixed Amount |  |  |  |  |  |  |  |  |  |  |  | $ 8,000,000
Retained Earnings, Unappropriated |  |  |  |  |  | $ 1,797,000,000 |  | $ 1,999,400,000 |  |  |  |
Common stock |  |  |  |  |  | $ 57,700,000 |  | $ 59,000,000 |  |  |  |
Spire Missouri [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 1,480,000 |  | 1,480,000 |  |  |  |
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized |  |  |  |  |  |  |  | $ 850,000,000 |  |  |  |
Preferred Stock, Shares Issued (in shares) |  |  |  |  |  | 0 |  | 0 |  |  |  |
Common stock |  |  |  |  |  | $ 100,000 |  | $ 100,000 |  |  |  |
Spire Missouri [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock |  |  |  |  |  |  | $ 74,400,000 |  |  |  |  |
Spire Alabama Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 120,000 |  | 120,000 |  |  |  |
Preferred Stock, Shares Issued (in shares) |  |  |  |  |  | 0 |  | 0 |  |  |  |
At The Market Program [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Forward Sales Agreements, Shares Settled (in shares) |  | 1,744,549 |  | 542,515 |  |  |  |  |  |  |  |
Proceeds from Shares Settled |  | $ 112,200,000 |  | $ 32,400,000 |  |  |  |  |  |  |  |
Sale of Stock, Maximum Aggregate Offering Price |  |  |  |  |  |  |  | $ 123,600,000 |  |  | $ 200,000,000 |
Forward Sales Agreements, Shares (in shares) |  |  |  |  |  | 663,619 |  |  |  |  |  |
Forward Sales Agreements, Proceeds If Settled |  |  |  |  |  |  |  |  | $ 42,400,000 |  |  |
First Mortgage Bond [Member] | Spire Missouri [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument face amount |  |  |  |  |  |  | 350,000,000 |  |  |  |  |
Parent Company [Member] | Two Thousand Twenty One Series A Zero Point Seven Five Percentage Remarketable Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument face amount |  |  |  |  | $ 1,000 |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 0.75% |  |  |  |  |  |  |
Parent Company [Member] | First Mortgage Bond [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument face amount |  |  |  |  |  |  | $ 150,000,000 |  |  |  |  |
Depositary Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 10,000,000 |  | 10,000,000 |  |  |  |
Preferred Stock, Shares Outstanding, Ending Balance (in shares) |  |  | 10,000,000 |  |  | 10,000,000 |  | 10,000,000 |  |  |  |
Fractional Interest In Share Of Series A Preferred Stock |  |  | 0.001% |  |  |  |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share (in dollars per share) |  |  |  |  |  | $ 25 |  | $ 25 |  |  |  |
Preferred Stock, Liquidation Preference Per Share (in dollars per share) |  |  | $ 25 |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Issued (in shares) |  |  |  |  |  | 10,000,000 |  | 10,000,000 |  |  |  |
Series A Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Dividend Rate, Percentage |  |  | 5.90% |  |  |  |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share (in dollars per share) |  |  | $ 25 |  |  |  |  |  |  |  |  |
Preferred Stock, Liquidation Preference Per Share (in dollars per share) |  |  | $ 25,000 |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues (in shares) |  |  | 10,000 |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Preferred Stock and Preference Stock |  |  | $ 242,000,000 |  |  |  |  |  |  |  |  |
Preferred Stock, Redemption Price Per Share (in dollars per share) |  |  |  |  |  |  |  | $ 25,000 |  |  |  |
Corporate Unit [Member] | Parent Company [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues (in shares) | 2,745,733 |  |  |  | 3,500,000 |  |  |  |  |  |  |
Equity Units, Stated Value (in dollars per share) |  |  |  |  | $ 50 |  |  |  |  |  |  |
Percentage of Undivided Beneficial Ownership Interest |  |  |  |  | 5.00% |  |  |  |  |  |  |
Equity Units, Applicable Market Value (in dollars per share) |  |  |  |  |  |  |  |  |  | $ 58.6809 |  |
Equity Units, Number of Common Stock Obligated to be Purchased After Adjustment (in shares) |  |  |  |  |  |  |  |  |  | 0.7845 |  |
Equity Stock Units Issued | $ 175,000,000 |  |  |  |  |  |  |  |  |  |  |

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