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ACQUISITIONS - Narrative (Details) - USD ($) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 12, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 12, 2017 | Sep. 30, 2014
Business Acquisition [Line Items] |  |  |  |  |  |
Payments to acquire businesses, net of cash acquired |  | $ 3.8 | $ (317.7) | $ 0.0 |  |
Goodwill |  | 1,171.6 | $ 1,164.9 | $ 946.0 |  | $ 937.8
EnergySouth |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Percentage of voting interest acquired | 100.00% |  |  |  |  |
Payments to acquire businesses, net of cash acquired | $ (313.9) |  |  |  |  |
Goodwill | $ 218.9 | $ 225.6 |  |  | $ 225.6 |

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