Information by Operating Segment - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting [Abstract]
Net Income	$ 271.7	$ 88.6	$ 184.6
Impairments	0.0	148.6	0.0
Provision for ISRS rulings	0.0	0.0	12.2
Missouri regulatory adjustments	(9.0)	0.0	0.0
Fair value and timing adjustments	3.3	2.5	1.2
Acquisition, divestiture and restructuring activities	(1.3)	0.0	0.4
Income tax effect of adjustments	1.6	(31.9)	(3.3)
Net Economic Earnings	$ 266.3	$ 207.8	$ 195.1