ACQUISITIONS - Narrative (Details) - USD ($) $ in Millions				12 Months Ended
	Sep. 12, 2016	Jan. 06, 2015	Aug. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Business Acquisition [Line Items]
Payments to acquire businesses, net of cash acquired				$ (317.7)	$ 0.0	$ 0.0
Goodwill				$ 1,164.9	$ 946.0	$ 937.8	$ 247.1
Alagasco
Business Acquisition [Line Items]
Percentage of voting interest acquired			100.00%
Payments to acquire businesses, net of cash acquired		$ (1,313.4)	$ (1,305.2)
Payments for final reconciliation of acquisition		$ (8.2)
Goodwill			735.8
Goodwill, expected to be tax deductible			$ 717.6
EnergySouth
Business Acquisition [Line Items]
Percentage of voting interest acquired	100.00%
Payments to acquire businesses, net of cash acquired	$ (317.7)
Goodwill	$ 218.9