Business Segment Information (Reconciliation of Operating Income) (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Operating income		$ 577.2	$ 516.5		$ 1,138.2	$ 971.5
Facility closure		0.0	0.0	$ (29.0)	0.0	(29.0)
Company headquarters relocation income (expense)		33.8	0.0		33.8	0.0
Segment Total
Segment Reporting Information [Line Items]
Operating income	[1]	543.4	516.5		1,104.4	1,000.5
Segment Reconciling Items
Segment Reporting Information [Line Items]
Facility closure		0.0	0.0		0.0	(29.0)
Company headquarters relocation income (expense)		$ 33.8	$ 0.0		$ 33.8	$ 0.0

[1] Refer to the Reconciliations to Consolidated Results section below.