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, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting [Abstract]
Net Income	$ 133.6	$ 154.6	$ 200.6	$ 221.9
Provision for ISRS rulings	2.2	0.0	4.8	0.0
Unrealized loss (gain) on energy-related derivatives	11.6	(9.1)	15.3	(11.3)
Acquisition, divestiture and restructuring activities	0.0	0.0	0.0	0.4
Income tax effect of adjustments	(3.4)	2.4	(4.9)	2.8
Net Economic Earnings	$ 144.0	$ 147.9	$ 215.8	$ 213.8