INFORMATION BY OPERATING SEGMENT - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Mar. 07, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting [Abstract]
Net Income		$ 98.2	$ 108.0	$ 214.2	$ 153.2
Missouri regulatory adjustments		30.6	0.0	30.6	0.0
Unrealized loss on energy-related derivative contracts		11.8	1.6	12.6	5.4
Lower of cost or market inventory adjustments		0.0	0.1	0.0	0.0
Realized gain on economic hedges prior to sale of the physical commodity		(0.2)	(0.1)	(0.3)	(0.2)
Acquisition, divestiture and restructuring activities		2.0	0.1	3.7	0.2
Income tax effect of adjustments		(11.1)	(0.7)	(11.7)	(2.1)
Effects of the Tax Cuts and Jobs Act	$ 33.0	5.9	0.0	(54.0)	0.0
Net Economic Earnings		$ 137.2	$ 109.0	$ 195.1	$ 156.5