ACQUISITIONS - Pro Forma Information, Revenues and Earnings (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Alagasco and EnergySouth
Business Acquisition [Line Items]
Total Operating Revenues	$ 1,632.4	$ 2,081.6	$ 2,187.1
Net Income	$ 153.9	$ 143.6	$ 133.5
Basic earnings (loss) per share (in dollars per share)	$ 3.48	$ 3.32	$ 3.11
Diluted earnings (loss) per share (in dollars per share)	$ 3.46	$ 3.31	$ 3.10
Alagasco
Business Acquisition [Line Items]
Pro forma, Total Operating Revenues	$ 368.5	$ 479.2	$ 19.7
Pro forma, Net Income (Loss)	$ 53.2	$ 48.0	$ (2.9)
Pro forma, Earnings (Loss) Per Share (in dollars per share)	$ 1.20	$ 1.11	$ (0.08)
EnergySouth
Business Acquisition [Line Items]
Pro forma, Total Operating Revenues	$ 3.3
Pro forma, Net Income (Loss)	$ (0.2)
Pro forma, Earnings (Loss) Per Share (in dollars per share)	$ 0.00