Information by Operating Segment - Reconciliation of Consolidated Net Economic Earnings to Consolidated Net Income (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting [Abstract]
Net Income (Loss)	$ 67.3	$ 116.0
Unrealized (gain) loss on energy-related derivatives	(2.2)	0.8
Realized gain on economic hedges prior to the sale of the physical commodity	0.0	(0.1)
Acquisition, divestiture and restructuring activities	0.4	1.7
Income tax effect of adjustments	0.4	(0.6)
Effect of the Tax Cuts and Jobs Act	0.0	(59.9)
Net Economic Earnings	$ 65.9	$ 57.9