INFORMATION BY OPERATING SEGMENT - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Mar. 07, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting [Abstract]
Net Income		$ 25.9	$ 21.7	$ 240.1	$ 174.9
Missouri regulatory adjustments		0.0	0.0	30.6	0.0
Unrealized (gain) loss on energy-related derivative contracts		(16.0)	(2.2)	(3.4)	3.2
Realized gain on economic hedges prior to sale of the physical commodity		0.0	0.0	(0.3)	(0.2)
Acquisition, divestiture and restructuring activities		3.3	1.9	7.0	2.1
Income tax effect of adjustments		2.0	0.2	(9.7)	(1.9)
Effects of the Tax Cuts and Jobs Act	$ 33.0	0.0	0.0	(54.0)	0.0
Net Economic Earnings		$ 15.2	$ 21.6	$ 210.3	$ 178.1