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Note 5 - Shareholders' Equity (Details Textual) - USD ($) |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended | 19 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 27, 2023 | May 21, 2019 | Feb. 28, 2021 | Jun. 30, 2023 | Mar. 31, 2023 | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Mar. 01, 2024 | Nov. 10, 2023 | Apr. 28, 2022
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 5,000,000 |  | 5,000,000 |  |  |
Number of Shares Authorized for Issuance (in shares) |  |  |  |  |  | 250,000 |  | 250,000 |  |  |
Number Of Shares Remaining and Available for Issuance (in shares) |  |  |  |  |  | 244,182 |  | 244,182 |  |  |
Retained Earnings Dividend Restrictions Formula Component Fixed Amount |  |  |  |  |  |  | $ 8,000,000 |  |  |  |
Retained Earnings, Unappropriated |  |  |  |  |  | $ 1,678,500,000 | 1,579,400,000 | $ 1,678,500,000 |  |  |
Stock Issued During Period, Value, New Issues |  |  |  |  |  | $ 40,500,000 | $ 50,300,000 |  |  |  |
Spire Missouri [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  |  | 1,480,000 |  |  |  |
Preferred Stock, Shares Outstanding, Ending Balance (in shares) |  |  |  |  |  | 0 | 0 | 0 |  |  |
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized |  |  |  |  |  | $ 800,000,000 |  | $ 800,000,000 |  |  |
Stock Issued During Period, Value, New Issues |  |  |  |  |  | $ 38,700,000 | $ 51,100,000 |  |  |  |
Preferred Stock, Shares Issued (in shares) |  |  |  |  |  | 0 | 0 | 0 |  |  |
Spire Alabama Inc [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 120,000 | 120,000 | 120,000 |  |  |
Preferred Stock, Shares Outstanding, Ending Balance (in shares) |  |  |  |  |  | 0 | 0 | 0 |  |  |
Preferred Stock, Shares Issued (in shares) |  |  |  |  |  | 0 | 0 | 0 |  |  |
Forecast [Member] |  |  |  |  |  |  |  |  |  |  |
Number Of Shares Remaining and Available for Issuance (in shares) |  |  |  |  |  |  |  |  |  | 237,891 |
At The Market Program [Member] |  |  |  |  |  |  |  |  |  |  |
Number of Shares Authorized for Issuance (in shares) |  |  |  |  |  | 17,900,000 |  | 17,900,000 |  |  | 200,000,000
Forward Sales Agreements, Shares (in shares) |  |  |  | 2,087,231 | 228,690 |  |  |  |  |  |
Forward Sales Agreements, Shares Settled (in shares) | 571,372 |  |  |  |  |  |  |  |  |  |
Proceeds from Shares Settled | $ 38,500,000 |  |  |  |  |  |  |  |  |  |
Forward Sales Agreements, Block Sales, Shares (in shares) |  |  |  |  |  | 1,744,549 |  |  |  |  |
Forward Sales Agreements, Proceeds If Settled |  |  |  |  |  | $ 111,200,000 |  | $ 111,200,000 |  |  |
Universal Shelf Registration [Member] | Spire Missouri [Member] |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Value, New Issues |  |  |  |  |  | $ 79,100,000 |  | 79,100,000 |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues (in shares) |  |  |  |  |  | 611,872 | 719,625 |  |  |  |
Stock Issued During Period, Value, New Issues |  |  |  |  |  | $ 600,000 | $ 700,000 |  |  |  |
Common Stock [Member] | Spire Missouri [Member] |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues (in shares) |  |  |  |  |  | 530 | 748 |  |  |  |
Stock Issued During Period, Value, New Issues |  |  |  |  |  | $ 0 | $ 0 |  |  |  |
First Mortgage Bond [Member] | Spire Missouri [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  | 400,000,000 |  | 400,000,000 |  |  |
Parent Company [Member] |  |  |  |  |  |  |  |  |  |  |
Present Value of Stock Purchase Contract Payment |  |  |  |  |  | $ 35,000,000 |  | $ 35,000,000 |  |  |
Parent Company [Member] | Common Stock [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Number of Shares to Be Purchased Per Stock Purchase Contract Settlement Rate (in shares) |  |  | 0.6354 |  |  |  |  |  |  |  |
Parent Company [Member] | Common Stock [Member] | Minimum [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |
Number of Shares to Be Purchased Per Stock Purchase Contract Settlement Rate (in shares) |  |  |  |  |  |  |  |  | 0.6354 |  |
Parent Company [Member] | Two Thousand Twenty One Series A Zero Point Seven Five Percentage Remarketable Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 1,000 |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  | 0.75% |  |  |  |  |  |  |  |
Stock Purchase Contracts Rate |  |  | 6.75% |  |  |  |  |  |  |  |
Redemption Prior to August Fifteen Two Thousand Twenty Four [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Redemption Price Per Share (in dollars per share) |  |  |  |  |  | $ 25,500 |  | $ 25,500 |  |  |
Redemption on Or After August Fifteen Two Thousand Twenty Four [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Redemption Price Per Share (in dollars per share) |  |  |  |  |  | $ 25,000 |  | $ 25,000 |  |  |
Depositary Shares [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  | 10,000,000 | 10,000,000 | 10,000,000 |  |  |
Preferred Stock, Shares Outstanding, Ending Balance (in shares) |  | 10,000,000 |  |  |  | 10,000,000 | 10,000,000 | 10,000,000 |  |  |
Preferred Stock, Share Interest Rate |  | 0.001% |  |  |  |  |  |  |  |  |
Preferred Stock, Dividend Rate, Percentage |  | 5.90% |  |  |  |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share (in dollars per share) |  | $ 25 |  |  |  | $ 25 | $ 25 | $ 25 |  |  |
Preferred Stock, Liquidation Preference Per Share (in dollars per share) |  | $ 25,000 |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Preferred Stock and Preference Stock |  | $ 242,000,000 |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Issued (in shares) |  |  |  |  |  | 10,000,000 | 10,000,000 | 10,000,000 |  |  |
Series A Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Dividend Rate, Percentage |  | 5.90% |  |  |  |  |  |  |  |  |
Corporate Unit [Member] | Parent Company [Member] |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues (in shares) |  |  | 3,500,000 |  |  |  |  |  |  |  |
Equity Stock Units Issued |  |  | $ 175,000,000 |  |  |  |  |  |  |  |
Proceeds from Issuance or Sale of Equity, Total |  |  | $ 169,300,000 |  |  |  |  |  |  |  |
Equity Units, Stated Value (in dollars per share) |  |  | $ 50 |  |  |  |  |  |  |  |
Percentage of Undivided Beneficial Ownership Interest |  |  | 5.00% |  |  |  |  |  |  |  |
Shares Issued, Price Per Share (in dollars per share) |  |  | $ 50 |  |  |  |  |  |  |  |

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