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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Sep. 30, 2016
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | $ 13.0 | $ 18.4
Fair value of liability derivatives presented above | 13.3 | 12.5
Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.7 |
Fair value of liability derivatives presented above | 1.1 |
Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 12.3 | 15.2
Fair value of liability derivatives presented above | 12.2 | 8.6
Gas Utility: | Operating Segments | Gasoline and heating oil contracts | Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.1 | 0.3
Fair value of liability derivatives presented above | 0.0 | 0.0
Gas Utility: | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 0.0
Fair value of liability derivatives presented above |  | 0.2
Gas Utility: | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 3.4 | 5.4
Fair value of liability derivatives presented above | 1.9 | 1.6
Gas Marketing: | Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 3.2
Fair value of liability derivatives presented above |  | 3.9
Gas Marketing: | Natural gas contracts | Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 2.5
Fair value of liability derivatives presented above |  | 0.8
Gas Marketing: | Interest rate swaps | Designated as Hedging Instrument | Current Liabilities – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.0 |
Fair value of liability derivatives presented above | 0.9 |
Gas Marketing: | Operating Segments | Natural gas contracts | Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.3 |
Fair value of liability derivatives presented above | 0.2 |
Gas Marketing: | Operating Segments | Natural gas contracts | Designated as Hedging Instrument | Deferred Charges – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.3 | 0.4
Fair value of liability derivatives presented above | 0.0 | 0.1
Gas Marketing: | Operating Segments | Interest rate swaps | Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 0.0
Fair value of liability derivatives presented above |  | 3.0
Gas Marketing: | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 1.7 | 0.8
Fair value of liability derivatives presented above | 1.4 | 4.1
Gas Marketing: | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Deferred Charges – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.3 | 0.0
Fair value of liability derivatives presented above | 0.5 | 0.1
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 5.3 | 6.5
Fair value of liability derivatives presented above | 0.1 | 0.2
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Charges – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.4 | 2.1
Fair value of liability derivatives presented above | 0.0 | 0.3
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Current Liabilities – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.8 | 0.2
Fair value of liability derivatives presented above | 5.0 | 2.0
Gas Marketing: | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Credits – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.4 | 0.2
Fair value of liability derivatives presented above | 3.3 | 0.1
Spire Missouri |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 3.5 | 5.7
Fair value of liability derivatives presented above | 1.9 | 1.8
Spire Missouri | Gas Utility: |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 3.5 | 5.7
Fair value of liability derivatives presented above | 1.9 | 1.8
Spire Missouri | Gas Utility: | Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 0.3
Fair value of liability derivatives presented above |  | 0.0
Spire Missouri | Gas Utility: | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 5.4
Fair value of liability derivatives presented above |  | 1.8
Spire Missouri | Gas Utility: | Gasoline and heating oil contracts | Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 0.1 |
Fair value of liability derivatives presented above | 0.0 |
Spire Missouri | Gas Utility: | Gasoline and heating oil contracts | Designated as Hedging Instrument | Accounts Receivable – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 0.3
Fair value of liability derivatives presented above |  | 0.0
Spire Missouri | Gas Utility: | Natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above |  | 0.0
Fair value of liability derivatives presented above |  | 0.2
Spire Missouri | Gas Utility: | Natural gas contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other |  |
Derivatives, Fair Value [Line Items] |  |
Fair value of asset derivatives presented above | 3.4 | 5.4
Fair value of liability derivatives presented above | $ 1.9 | $ 1.6

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