Revenue Recognition - Dual Reporting - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income (loss)	$ 346.0	$ 227.4	$ 426.3	$ (9.0)
Currency translation adjustments	19.0	$ 76.5	(9.5)	$ 60.4
Impact of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income (loss)	(4.8)		(2.5)
Currency translation adjustments	0.0		0.2
Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income (loss)	341.2		423.8
Currency translation adjustments	$ 19.0		$ (9.3)