Information by Operating Segment - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting [Abstract]
Net Income	$ (34.3)	$ (3.0)	$ 154.6	$ 67.3	$ (25.9)	$ 25.9	$ 98.2	$ 116.0	$ 184.6	$ 214.2	$ 161.6
Provision for ISRS rulings									12.2	0.0	0.0
Missouri regulatory adjustments									0.0	30.6	0.0
Unrealized loss (gain) on energy-related derivatives									1.2	(4.0)	6.0
Realized gain on economic hedges prior to the sale of the physical commodity									0.0	(0.3)	(0.3)
Acquisition, divestiture and restructuring activities									0.4	13.6	4.0
Income tax effect of adjustments									(3.3)	(10.3)	(3.7)
Effect of the Tax Cuts and Jobs Act									0.0	(60.1)	0.0
Net Economic Earnings									$ 195.1	$ 183.7	$ 167.6