ACQUISITIONS - Summary of Consideration Paid, Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions			12 Months Ended
	Sep. 30, 2017	Sep. 12, 2016	Sep. 30, 2017	Sep. 30, 2015	Sep. 12, 2017	Sep. 30, 2016	Sep. 30, 2014
Business Acquisition [Line Items]
Goodwill	$ 1,171.6		$ 1,171.6	$ 946.0		$ 1,164.9	$ 937.8
Measurement period adjustments
Goodwill			6.7	$ 8.2
EnergySouth
Business Acquisition [Line Items]
Utility plant	199.5	$ 199.5	199.5
Cash	2.0	2.0	2.0
Other current assets	17.7	17.5	17.7
Other assets	69.1	79.8	69.1
Long-term debt	(67.0)	(67.0)	(67.0)
Other current liabilities	(42.7)	(42.7)	(42.7)
Deferred tax liabilities	(35.5)	(35.5)	(35.5)
Other liabilities	(52.8)	(52.8)	(52.8)
Total identifiable net assets	90.3	100.8	90.3
Goodwill	225.6	218.9	$ 225.6		$ 225.6
Measurement period adjustments
Other current assets		0.2
Other assets		(10.7)
Total identifiable net assets		(10.5)
Goodwill		6.7
Consideration (cash)		(3.8)
Consideration (cash)	$ 315.9	$ 319.7