Business Segment Information (Reconciliation of Operating Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Segment Reporting Information [Line Items]
Business separation costs	$ 0.0	$ 0.0	$ 0.0	$ (32.5)
Cost reduction and asset actions	0.0	(10.3)	0.0	(60.3)	$ (151.4)
Operating Income	455.4	395.6	916.1	723.9
Segment Total
Segment Reporting Information [Line Items]
Operating Income	455.4	405.9	916.1	816.7
Segment Reconciling Items
Segment Reporting Information [Line Items]
Business separation costs	0.0	0.0	0.0	(32.5)
Cost reduction and asset actions	$ 0.0	$ (10.3)	$ 0.0	$ (60.3)