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STOCKHOLDERS' EQUITY - Narrative (Details) $ / shares in Units, equity_unit in Thousands |  |  |  | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 15, 2017 USD ($) | Apr. 03, 2017 $ / shares shares | May 17, 2016 USD ($) shares | Sep. 30, 2017 USD ($) shares | Sep. 30, 2016 USD ($) shares | Sep. 30, 2015 USD ($) | Dec. 31, 2014 USD ($) equity_unit | May 30, 2017 USD ($) | Nov. 11, 2016 shares | Feb. 10, 2016 USD ($) | Jun. 30, 2014 $ / shares | Jun. 20, 2014 shares
Accumulated other comprehensive income (loss), net of income taxes [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of equity units issued (in shares) | equity_unit |  |  |  |  |  |  | 2,875 |  |  |  |  |
Stated value per Corporate Unit (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 |
Interest rate on Subordinated borrowing |  |  |  |  |  |  | 2.00% |  |  |  |  |
Issuance of stock (in shares) |  |  | 2,185,000 |  |  |  |  |  |  |  |  |
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) |  |  |  | 244,130 |  |  |  |  | 239,945 |  |  |
Issuance of common stock | $ |  |  | $ 133,200,000 | $ 145,500,000 | $ 133,200,000 |  |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  |  | 5,000,000 | 5,000,000 |  |  |  |  |  |  |
Preferred stock shares issued (in shares) |  |  |  | 0 | 0 |  |  |  |  |  |  |
Preferred stock shares outstanding (in shares) |  |  |  | 0 | 0 |  |  |  |  |  |  |
Dividend payment restriction formula component - fixed amount | $ |  |  |  | $ 8,000,000.0 |  |  |  |  |  |  |  |
Retained earnings available to pay dividends in period | $ |  |  |  | $ 1,010,800,000 | $ 916,800,000 |  |  |  |  |  |  |
Sale of Stock, Number of Shares Issued in Transaction |  | 2,504,684 |  |  |  |  |  |  |  |  |  |
Sale of Stock, Price Per Share | $ / shares |  | $ 57.3921 |  |  |  |  |  |  |  |  |  |
Share Purchase Obligation, Conversion to Common Stock, Rate Per Share |  | 0.8712 |  |  |  |  |  |  |  |  |  |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |
Accumulated other comprehensive income (loss), net of income taxes [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  |  | 1,480,000 | 1,480,000 |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized | $ |  |  |  | $ 300,000,000 |  |  |  | $ 300,000,000.0 |  | $ 300,000,000.0 |  |
Proceeds from Issuance of Debt | $ | $ 170,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Regulatory Preferred Stock and Debt Securities Issuance, Remaining Authorized Amount | $ |  |  |  | $ 130,000,000 |  |  |  |  |  |  |  |
Spire Missouri | Affiliated Entity |  |  |  |  |  |  |  |  |  |  |  |
Accumulated other comprehensive income (loss), net of income taxes [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of stock (in shares) |  |  |  |  | 0 |  |  |  |  |  |  |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |
Accumulated other comprehensive income (loss), net of income taxes [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock | $ |  |  |  |  |  | $ (27,000,000) |  |  |  |  |  |
Preferred stock shares authorized (in shares) |  |  |  | 120,000 | 120,000 |  |  |  |  |  |  |
Preferred stock shares issued (in shares) |  |  |  | 0 | 0 |  |  |  |  |  |  |
Preferred stock shares outstanding (in shares) |  |  |  | 0 | 0 |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized | $ |  |  |  | $ 75,000,000.0 |  |  |  |  |  |  |  |
Junior Subordinated Debt |  |  |  |  |  |  |  |  |  |  |  |
Accumulated other comprehensive income (loss), net of income taxes [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from convertible debt | $ |  |  |  |  |  |  | $ 143,800,000 |  |  |  |  |

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