SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) $ in Millions	9 Months Ended
	Jun. 30, 2017 USD ($)
Goodwill [Roll Forward]
Balance as of September 30, 2016	$ 1,164.9
Adjustments related to the acquisition of EnergySouth	(1.0)
Balance as of June 30, 2017	1,163.9
Operating Segments | Gas Utility
Goodwill [Roll Forward]
Balance as of September 30, 2016	210.2
Adjustments related to the acquisition of EnergySouth	0.0
Balance as of June 30, 2017	210.2
Operating Segments | Gas Marketing
Goodwill [Roll Forward]
Balance as of September 30, 2016	0.0
Adjustments related to the acquisition of EnergySouth	0.0
Balance as of June 30, 2017	0.0
Other
Goodwill [Roll Forward]
Balance as of September 30, 2016	954.7
Adjustments related to the acquisition of EnergySouth	(1.0)
Balance as of June 30, 2017	$ 953.7