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INCOME TAXES - Impact of Tax Cuts and Jobs Act (Details) - USD ($) $ in Millions | 3 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Deferred Income Tax Assets And Liabilities [Line Items] |  |
Adjustment to deferred tax assets | $ 0.0 |
Adjustment to deferred tax liabilities | (296.6) |
Adjustment to deferred income tax expense | (59.9) | $ 0.0
Adjustment to regulatory assets | (59.4) |
Adjustment to regulatory liabilities | 177.3 |
Spire Missouri |  |
Deferred Income Tax Assets And Liabilities [Line Items] |  |
Adjustment to deferred tax assets | 0.0 |
Adjustment to deferred tax liabilities | (264.1) |
Adjustment to deferred income tax expense | (43.9) |
Adjustment to regulatory assets | (61.0) |
Adjustment to regulatory liabilities | 159.2 |
Spire Alabama |  |
Deferred Income Tax Assets And Liabilities [Line Items] |  |
Adjustment to deferred tax assets | (60.8) |
Adjustment to deferred tax liabilities | 0.0 |
Adjustment to deferred income tax expense | 59.2 |
Adjustment to regulatory assets | 1.6 |
Adjustment to regulatory liabilities | $ 0.0 |

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