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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions |  | 3 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) reporting_unit | Sep. 30, 2017 USD ($) | Dec. 31, 2016 USD ($)
Related Party Transaction [Line Items] |  |  |  |
Number of operating segments | reporting_unit |  | 2 |  |
Spire Missouri |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Accrued unbilled revenues | $ 116.2 | $ 116.2 | $ 30.1 | $ 103.5
Spire Alabama |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Accrued unbilled revenues | $ 13.2 | $ 13.2 | $ 1.9 | $ 22.0
Ryckman Creek Resources, LLC |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Voting interest acquired | 80.00% | 80.00% |  |
Purchase price of acquisition | $ 26.0 |  |  |
Cash paid for acquisition | 16.0 |  |  |
Promissory note for acquisition | $ 10.0 |  |  |

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