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LONG-TERM DEBT - Narrative (Details) $ / shares in Units, equity_unit in Thousands |  |  |  |  | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 15, 2017 USD ($) | Apr. 03, 2017 USD ($) equity_unit $ / shares shares | Feb. 22, 2017 USD ($) | Jun. 11, 2014 USD ($) $ / shares | Sep. 30, 2017 USD ($) | Dec. 31, 2014 USD ($) equity_unit | Sep. 30, 2014 equity_unit | May 30, 2017 USD ($) | Mar. 15, 2017 USD ($) | Mar. 10, 2017 USD ($) | Sep. 30, 2016 | Feb. 10, 2016 USD ($) | Jun. 30, 2014 $ / shares
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 2,112,000,000 |  |  |  |  |  |  |  |
Interest rate on Subordinated borrowing |  |  |  |  |  | 2.00% |  |  |  |  |  |  |
Number of equity units issued (in shares) | equity_unit |  |  |  |  |  | 2,875 |  |  |  |  |  |  |
Stated value per Corporate Unit (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000
Percent of common stock equity |  |  |  |  | 48.70% |  |  |  |  |  | 46.10% |  |
Percent of long-term debt |  |  |  |  | 51.30% |  |  |  |  |  | 53.90% |  |
Sale of Stock, Number of Shares Issued in Transaction | shares |  | 2,504,684 |  |  |  |  |  |  |  |  |  |  |
Sale of Stock, Price Per Share | $ / shares |  | $ 57.3921 |  |  |  |  |  |  |  |  |  |  |
Payment to Purchase Share, Price Per Share | $ / shares |  | $ 67.50 |  |  |  |  |  |  |  |  |  |  |
Share Purchase Obligation, Conversion to Common Stock, Rate Per Share |  | 0.8712 |  |  |  |  |  |  |  |  |  |  |
Repayments of Short-term Debt |  | $ 142,000,000 |  |  |  |  |  |  |  |  |  |  |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued amount |  |  | $ 6,200,000 |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt |  |  | 150,000,000 |  |  |  |  |  |  |  |  |  |
Senior Notes | Senior Notes Due March 15, 2027 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued amount |  |  |  |  |  |  |  |  | $ 100,000,000.0 |  |  |  |
Stated interest rate on debt issued |  |  |  |  |  |  |  |  | 3.93% |  |  |  |
Senior Notes | Senior Notes Due in 2024 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued amount |  |  | $ 143,800,000.0 |  |  |  |  |  |  |  |  |  |
Stated interest rate on debt issued |  |  | 3.543% |  |  |  |  |  |  |  |  |  |
Senior Notes | One to six months prior to maturity | Subject to Callable at Par |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redemption period prior to maturity, minimum |  |  |  |  | 1 month |  |  |  |  |  |  |  |
Redemption period prior to maturity, maximum |  |  |  |  | 6 months |  |  |  |  |  |  |  |
Bonds | Bond Due September Fifteen Twenty Thirty Two [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued amount | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate on debt issued | 3.68% |  |  |  |  |  |  |  |  |  |  |  |
Bonds | Bond Due September Fifteen Twenty Forty Seven [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued amount | $ 70,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate on debt issued | 4.23% |  |  |  |  |  |  |  |  |  |  |  |
Bonds | Bond Due September Fifteen Twenty Fifty Seven [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt issued amount | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate on debt issued | 4.38% |  |  |  |  |  |  |  |  |  |  |  |
Junior Subordinated Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from convertible debt |  |  |  |  |  | $ 143,800,000 |  |  |  |  |  |  |
Notes Payable to Banks |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchase Amount |  |  |  |  |  |  |  |  |  | $ 250,000,000.0 |  |  |
Other subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 67,000,000 |  |  |  |  |  |  |  |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | 980,000,000 |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 975,000,000 |  |  |  |  |  |  |  |
Percent of common stock equity |  |  |  |  | 54.60% |  |  |  |  |  | 57.10% |  |
Percent of long-term debt |  |  |  |  | 45.40% |  |  |  |  |  | 42.90% |  |
Spire Missouri | No Call Options |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 25,000,000 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | 25,000,000 |  |  |  |  |  |  |  |
Spire Missouri | Make Whole Call Options |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | 1,037,000,000 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | 435,000,000 |  |  |  |  |  |  |  |
Spire Missouri | Subject to Callable Currently [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | 5,000,000 |  |  |  |  |  |  |  |
Spire Missouri | Subject to Callable at Par |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | 1,045,000,000 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 520,000,000 |  |  |  |  |  |  |  |
Spire Missouri | Three to six months prior to maturity | Subject to Callable at Par |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redemption period prior to maturity, minimum |  |  |  |  | 3 months |  |  |  |  |  |  |  |
Redemption period prior to maturity, maximum |  |  |  |  | 6 months |  |  |  |  |  |  |  |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 250,000,000 |  |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized |  |  |  |  | 75,000,000.0 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 250,000,000 |  |  |  |  |  |  |  |
Percent of common stock equity |  |  |  |  | 77.80% |  |  |  |  |  | 77.80% |  |
Percent of long-term debt |  |  |  |  | 22.20% |  |  |  |  |  | 22.20% |  |
Spire Alabama | Make Whole Call Options |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | $ 250,000,000 |  |  |  |  |  |  |  |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on Subordinated borrowing |  |  |  | 2.00% |  |  |  |  |  |  |  |  |
Number of equity units issued (in shares) | equity_unit |  | 2,875 |  |  |  |  | 2,875 |  |  |  |  |  |
Stated value per Corporate Unit (in dollars per share) | $ / shares |  |  |  | $ 50 |  |  |  |  |  |  |  |  |
Spire Alabama | Junior Subordinated Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from convertible debt |  |  |  | $ 143,800,000 |  |  |  |  |  |  |  |  |

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