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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting [Abstract]
Net (Loss) Income	$ (92.3)	$ (3.0)	$ 108.3	$ 218.9
Impairments	148.6	0.0	148.6	0.0
Provision for ISRS rulings	(4.8)	0.0	0.0	0.0
Unrealized (gain) loss on energy-related derivatives	(18.5)	8.0	(3.2)	(3.3)
Lower of cost or market inventory adjustments	0.0	2.7	0.0	2.7
Acquisition, divestiture and restructuring activities	0.0	0.0	0.0	0.4
Income tax effect of adjustments	(25.7)	(2.7)	(30.6)	0.1
Net Economic Earnings	$ 7.3	$ 5.0	$ 223.1	$ 218.8