Information by Operating Segment - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting [Abstract]
Net (Loss) Income	$ (3.0)	$ 25.9	$ 218.9	$ 240.1
Missouri regulatory adjustments	0.0	0.0	0.0	30.6
Unrealized loss (gain) on energy-related derivatives	8.0	(16.0)	(3.3)	(3.4)
Realized gain on economic hedges prior to the sale of the physical commodity	0.0	0.0	0.0	(0.3)
Lower of cost or market inventory adjustments	2.7	0.0	2.7	0.0
Acquisition, divestiture and restructuring activities	0.0	3.3	0.4	7.0
Income tax effect of adjustments	(2.7)	2.0	0.1	(9.7)
Effect of the Tax Cuts and Jobs Act	0.0	0.0	0.0	(54.0)
Net Economic Earnings	$ 5.0	$ 15.2	$ 218.8	$ 210.3