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ACQUISITIONS - Pro Forma Information, Revenues and Earnings (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Alagasco and EnergySouth |  |  |
Business Acquisition [Line Items] |  |  |
Total Operating Revenues | $ 1,740.7 | $ 1,632.4 | $ 2,081.6
Net Income | $ 161.6 | $ 153.9 | $ 143.6
Basic earnings (loss) per share (in dollars per share) | $ 3.44 | $ 3.48 | $ 3.32
Diluted earnings (loss) per share (in dollars per share) | $ 3.43 | $ 3.46 | $ 3.31
EnergySouth |  |  |
Business Acquisition [Line Items] |  |  |
Pro forma, Total Operating Revenues | $ 95.5 | $ 3.3 |
Pro forma, Net Income (Loss) | $ 9.4 | $ (0.2) |
Pro forma, Earnings (Loss) Per Share (in dollars per share) | $ 0.20 | $ 0.00 |

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