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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES Narrative (Details) gal in Millions, MMBTU in Millions |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
May 21, 2015 USD ($) | Sep. 30, 2017 USD ($) MMBTU $ / gal gal | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Dec. 01, 2015 USD ($) | Sep. 15, 2015 USD ($) | Apr. 30, 2015 USD ($) | Mar. 31, 2015 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) | MMBTU |  | 202.1 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2018 | MMBTU |  | 156.8 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2019 | MMBTU |  | 34.3 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2020 | MMBTU |  | 5.9 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2021 | MMBTU |  | 4.1 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2022 | MMBTU |  | 0.9 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2023 | MMBTU |  | 0.1 |  |  |  |  |  |
Unrealized gain on energy-related derivatives |  | $ (6,000,000) | $ 100,000 | $ 2,800,000 |  |  |  |
Gain (loss) to be reclassified within twelve months |  | 0 |  |  |  |  |  |
Cash margin receivables not offset with derivatives |  | $ 4,000,000 | 2,900,000 |  |  |  |  |
Spire Alabama |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Unsecured debt |  |  |  | 115,000,000 |  |  |  | $ 115,000,000
Gasoline futures contracts notional amount (in gallons) | gal |  | 0.1 |  |  |  |  |  |
Average price of future contracts (dollars per gallon) | $ / gal |  | 1.28 |  |  |  |  |  |
Spire Alabama | 3.21% Notes, due September 15, 2025 |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Unsecured debt |  |  |  |  |  | $ 35,000,000 |  |
Spire Alabama | Notes 4.31%, Due December 1, 2045 |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Unsecured debt |  |  |  |  | $ 80,000,000 |  |  |
Spire Missouri |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
MMBtu (millions) | MMBTU |  | 33.9 |  |  |  |  |  |
Gain (loss) to be reclassified within twelve months |  | $ 0 |  |  |  |  |  |
Cash margin receivables not offset with derivatives |  | $ 4,000,000 | 500,000 |  |  |  |  |
Gasoline futures contracts notional amount (in gallons) | gal |  | 0.3 |  |  |  |  |  |
Average price of future contracts (dollars per gallon) | $ / gal |  | 1.27 |  |  |  |  |  |
Designated as Hedging Instrument | Interest rate swaps | Spire Alabama |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Loss on termination and settlement of interest rate swap |  | $ 900,000 |  |  |  |  |  |
Gain on settlement of derivative | $ 2,700,000 | 14,100,000 |  | 2,700,000 |  |  |  |
Designated as Hedging Instrument | Interest rate swaps | Spire Energy South Inc |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Loss on termination and settlement of interest rate swap |  |  | 400,000 |  |  |  |  |
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional amount |  |  |  |  |  |  | $ 24,000,000.0 |
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Alabama |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional amount |  | $ 60,000,000 | 225,000,000 | $ 104,500,000 |  |  |  | $ 80,500,000
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Alabama | Minimum |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative fixed interest rate |  | 265.80% |  |  |  |  |  | 2.18%
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Alabama | Maximum |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative fixed interest rate |  |  |  |  |  |  |  | 2.85%
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps | Spire Energy South Inc |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional amount |  |  | $ 85,000,000 |  |  |  |  |

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