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Shareholders' Equity - Additional Information (Details) - USD ($) |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
May 21, 2019 | Feb. 06, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Nov. 13, 2020 | Mar. 24, 2020
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  | 5,000,000 | 5,000,000 |  |  |
Percentage of preferred stock | 5.90% |  |  |  |  |  |
Preferred stock, liquidation preference per share | $ 25,000 |  |  |  |  |  |
Net proceeds, after deducting commissions and sale expenses |  |  |  | $ 242,000,000.0 |  |  |
Preferred stock, redemption date |  |  | Aug. 15, 2024 |  |  |  |
Preferred stock voting rights |  |  | Shareholders of the Preferred Stock generally have no voting rights with respect to matters that generally require the approval of voting stockholders. The limited voting rights of holders of the Preferred Stock include the right to vote on certain matters that may affect the preference or special rights of the Preferred Stock. In addition, if and whenever dividends on any shares of Preferred Stock have not been declared and paid for at least six dividend periods, whether or not consecutive, the number of directors then constituting the Board shall automatically be increased by two (to be elected by the holders of the Preferred Stock) until all accumulated and unpaid dividends on the Preferred Stock have been paid in full. |  |  |  |
Sale of common stock |  |  | $ 32,000,000.0 | 14,400,000 | $ 153,000,000.0 |  |
Issuance of common stock |  |  | $ 41,100,000 | 19,500,000 | $ 154,700,000 |  |
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares) |  |  | 250,000 |  |  |  |
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares) |  |  | 207,254 |  |  |  |
Dividend payment restriction formula component - fixed amount |  |  | $ 8,000,000.0 |  |  |  |
Retained earnings available to pay dividends in period |  |  | $ 1,269,400,000 | $ 1,182,400,000 |  |  |
Spire Missouri |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  | 1,480,000 | 1,480,000 |  |  |
Preferred stock shares outstanding (in shares) |  |  | 0 | 0 |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized |  |  | $ 500,000,000.0 |  |  |  |
Regulatory preferred stock and debt securities issuance, remaining authorized amount |  |  | $ 125,000,000.0 |  |  |  |
Preferred stock shares issued (in shares) |  |  | 0 | 0 |  |  |
Spire Alabama |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  | 120,000 | 120,000 |  |  |
Preferred stock shares outstanding (in shares) |  |  | 0 | 0 |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized |  |  |  |  |  |  | $ 150,000,000.0
Preferred stock shares issued (in shares) |  |  | 0 | 0 |  |  |
Subsequent Event |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares) |  |  |  |  |  | 201,148 |
ATM Program |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Sale of common stock |  | $ 150,000,000.0 |  |  |  |  |
Number of shares issued |  |  | 446,619 | 179,630 |  |  |
Issuance of common stock |  |  | $ 32,000,000.0 | $ 14,400,000 |  |  |
Redemption prior to August 15, 2024 |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock redemption price per share |  |  | $ 25,500 |  |  |  |
Redemption on or after August 15, 2024 |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock redemption price per share |  |  | $ 25,000 |  |  |  |
Depositary Shares |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  | 10,000,000.0 | 10,000,000.0 |  |  |
Preferred stock shares outstanding (in shares) | 10,000,000 |  | 10,000,000.0 | 10,000,000.0 |  |  |
Preferred stock share interest rate | 0.10% |  |  |  |  |  |
Preferred stock, par value | $ 25.00 |  | $ 25.00 | $ 25.00 |  |  |
Net proceeds, after deducting commissions and sale expenses | $ 242,000,000.0 |  |  |  |  |  |
Preferred stock shares issued (in shares) |  |  | 10,000,000.0 | 10,000,000.0 |  |  |

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