Derivative Instruments - Pre-tax Amount of Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [Abstract]
Pre-tax amount of (losses) gains reclassified from accumulated other comprehensive income into the Consolidated Statement of Operations related to derivative forward exchange contracts designated as cash flow hedges	$ 16.7	$ (13.6)	$ (3.0)
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments [Abstract]
Pre-tax amount of gains (losses) from forward exchange contracts not designated as hedging instruments recognized in the Consolidated Statement of Operations	1.2	0.9	(10.4)
Forward exchange contracts
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net [Abstract] (Deprecated 2018-01-31)
Pre-tax (losses) gains recorded in other comprehensive income related to cash flow hedges	29.5	11.8	(16.1)
Pre-tax gains (losses) recorded in other comprehensive income related to net investment hedges	(4.9)	1.1	(16.3)
Treasury locks
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net [Abstract] (Deprecated 2018-01-31)
Pre-tax (losses) gains recorded in other comprehensive income related to cash flow hedges	(35.7)	0.0	0.0
Sales
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [Abstract]
Pre-tax amount of (losses) gains reclassified from accumulated other comprehensive income into the Consolidated Statement of Operations related to derivative forward exchange contracts designated as cash flow hedges	1.0	2.4	0.3
Cost of sales
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [Abstract]
Pre-tax amount of (losses) gains reclassified from accumulated other comprehensive income into the Consolidated Statement of Operations related to derivative forward exchange contracts designated as cash flow hedges	18.2	(17.2)	(2.8)
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments [Abstract]
Pre-tax amount of gains (losses) from forward exchange contracts not designated as hedging instruments recognized in the Consolidated Statement of Operations	(0.4)	1.0	(1.8)
Selling, general and administrative expenses
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [Abstract]
Pre-tax amount of (losses) gains reclassified from accumulated other comprehensive income into the Consolidated Statement of Operations related to derivative forward exchange contracts designated as cash flow hedges	(1.3)	1.2	(0.5)
Interest income (expense)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [Abstract]
Pre-tax amount of (losses) gains reclassified from accumulated other comprehensive income into the Consolidated Statement of Operations related to derivative forward exchange contracts designated as cash flow hedges	(1.2)	0.0	0.0
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Pre-tax amount of net gains recognized within the Consolidated Statement of Operations related to derivative instruments designated as fair value hedges, which fully offset the related net gains on the hedged debt instruments	30.9	(19.3)	(24.1)
Other income (expense)
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments [Abstract]
Pre-tax amount of gains (losses) from forward exchange contracts not designated as hedging instruments recognized in the Consolidated Statement of Operations	$ 1.6	$ (0.1)	$ (8.6)