Income Taxes Valuation Allowance Against Deferred Tax Assets (Details) - Valuation Allowance of Deferred Tax Assets [Member] - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Movement in Deferred Tax Asset Valuation Allowance [Roll Forward]
Balance at beginning of fiscal year		$ 218.0	$ 229.4	$ 219.1
Increases		46.2	25.4	50.8
Allowances related to purchase accounting	[1]		0.8	0.1
Reductions		(6.7)	(37.6)	(40.6)
Balance at end of fiscal year		$ 257.5	$ 218.0	$ 229.4

[1] Amounts in fiscal 2019 relate to the KapStone Acquisition. Amounts in fiscal 2018 relate to the MPS Acquisition.