ACQUISITIONS - Pro Forma Information, Revenues and Earnings (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	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