Information by Operating Segment - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting [Abstract]
Net Income	$ 173.6	$ 187.4	$ 229.3	$ 276.3
Missouri regulatory adjustment		(9.0)		(9.0)
Fair value and timing adjustments	9.9	20.1	13.6	4.1
Income tax adjustments	(2.5)	(2.9)	0.7	1.1
Net Economic Earnings	$ 181.0	$ 195.6	$ 243.6	$ 272.5