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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts and Sales Taxes (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Schedule of Gross Receipts Taxes [Line Items] |  |  |
Gross Receipts taxes recorded in regulated gas distribution operating revenues | $ 84.6 | $ 75.5 | $ 97.3
Spire Missouri |  |  |
Schedule of Gross Receipts Taxes [Line Items] |  |  |
Gross Receipts taxes recorded in regulated gas distribution operating revenues | 60.7 | 57.4 | 74.5
Spire Alabama |  |  |
Schedule of Gross Receipts Taxes [Line Items] |  |  |
Gross Receipts taxes recorded in regulated gas distribution operating revenues | $ 19.5 | $ 17.9 | $ 22.6

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- Definition Amount of gross receipts taxes associated with natural gas utility service, which are imposed on the Utility and billed to its customers. + References No definition available. + Details
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- Definition [Line Items] for Schedule of Gross Receipts Taxes [Table] + References No definition available. + Details
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- Details
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- Details
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