SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2018
Goodwill [Roll Forward]
Beginning balance	$ 1,164.9	$ 946.0
Goodwill		218.9
Acquisition	6.7
Ending balance	1,171.6	1,164.9
Goodwill	1,164.9	946.0	$ 1,171.6
Operating Segments | Gas Utility
Goodwill [Roll Forward]
Beginning balance	210.2	210.2
Goodwill		0.0
Acquisition	0.0
Ending balance	210.2	210.2
Goodwill	210.2	210.2	210.2
Operating Segments | Gas Marketing
Goodwill [Roll Forward]
Beginning balance	0.0	0.0
Goodwill		0.0
Acquisition	0.0
Ending balance	0.0	0.0
Goodwill	0.0	0.0	0.0
Other
Goodwill [Roll Forward]
Beginning balance	954.7	735.8
Goodwill		218.9
Acquisition	6.7
Ending balance	961.4	954.7
Goodwill	$ 954.7	$ 735.8	$ 961.4