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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting [Abstract]
Net Income (Loss)	$ 5.3	$ (92.3)	$ 281.6	$ 108.3
Impairments	0.0	148.6	0.0	148.6
Missouri regulatory adjustment	0.0	0.0	(9.0)	0.0
Provision for ISRS rulings	0.0	(4.8)	0.0	0.0
Fair value and timing adjustments	2.1	(18.5)	6.2	(3.2)
Income tax effect of adjustments	(0.5)	(25.7)	0.6	(30.6)
Net Economic Earnings (Loss)	$ 6.9	$ 7.3	$ 279.4	$ 223.1