SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) $ in Millions	3 Months Ended
	Dec. 31, 2016 USD ($)
Goodwill [Roll Forward]
Balance as of September 30, 2016	$ 1,164.9
Adjustments related to the acquisition of EnergySouth	(3.5)
Balance as of December 31, 2016	1,161.4
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 December 31, 2016	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 December 31, 2016	0.0
Other
Goodwill [Roll Forward]
Balance as of September 30, 2016	954.7
Adjustments related to the acquisition of EnergySouth	(3.5)
Balance as of December 31, 2016	$ 951.2