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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) - USD ($) $ in Millions | 3 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Schedule of Gross Receipts Taxes [Line Items] |  |
Gross Receipts taxes recorded in regulated gas distribution operating revenues | $ 23.1 | $ 19.4
Spire Missouri |  |
Schedule of Gross Receipts Taxes [Line Items] |  |
Gross Receipts taxes recorded in regulated gas distribution operating revenues | 16.2 | 14.1
Spire Alabama |  |
Schedule of Gross Receipts Taxes [Line Items] |  |
Gross Receipts taxes recorded in regulated gas distribution operating revenues | $ 5.6 | $ 4.2

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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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