INFORMATION BY OPERATING SEGMENT - Reconciliation of Consolidated Net Income to Consolidated Net Economic Earnings (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting [Abstract]
Net income	$ (25.9)	$ 25.9	$ 98.2	$ 116.0	$ (13.3)	$ 21.7	$ 108.0	$ 45.2	$ 214.2	$ 161.6	$ 144.2
Missouri regulatory adjustments									30.6
Unrealized (gain) loss on energy-related derivatives									(4.0)	6.0	(0.1)
Lower of cost or market inventory adjustments									0.0	0.0	0.2
Realized gain on economic hedges prior to the sale of the physical commodity									(0.3)	(0.3)	(1.6)
Acquisition, divestiture and restructuring activities									13.6	4.0	9.2
Income tax effect of adjustments									(10.3)	(3.7)	(2.8)
Effect of the Tax Cuts and Jobs Act									(60.1)	0.0	0.0
Net Economic Earnings									$ 183.7	$ 167.6	$ 149.1