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, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting [Abstract]
Net Income	$ (19.7)	$ (92.3)	$ 133.6	$ 67.0	$ (34.3)	$ (3.0)	$ 154.6	$ 67.3	$ 88.6	$ 184.6	$ 214.2
Impairments									148.6	0.0	0.0
Provision for ISRS rulings									0.0	12.2	0.0
Missouri regulatory adjustments									0.0	0.0	30.6
Fair value and timing adjustments									2.5	1.2	(4.3)
Acquisition, divestiture and restructuring activities									0.0	0.4	13.6
Income tax effect of adjustments									(31.9)	(3.3)	(10.3)
Effect of the Tax Cuts and Jobs Act									0.0	0.0	(60.1)
Net Economic Earnings									$ 207.8	$ 195.1	$ 183.7