Goodwill and Intangibles, net - Summary of Changes in Carrying Amounts of Goodwill and Intangible Assets (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Goodwill And Intangible Assets [Line Items]
Cost	$ 875.3		$ 875.3		$ 884.8
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Goodwill			2,277.1
Beginning Balance, Indefinite Lived Intangible Assets			1,024.3
Beginning Balance, Finite Lived Intangible Assets			587.7
Beginning Balance, Intangible Assets, Excluding Goodwill			1,612.0
Less: Components of amortization:
Effect of translation, Goodwill			3.1
Effect of translation, Finite Lived intangible assets			6.9
Ending Balance, Goodwill	2,280.2		2,280.2
Ending Balance, Indefinite Lived Intangible Assets	1,029.0		1,029.0
Ending Balance, Finite Lived Intangible Assets	560.5		560.5
Ending Balance, Intangible Assets, Excluding Goodwill	1,589.5		1,589.5
Finite-Lived Intangible Assets, Accumulated Amortization	314.8		314.8		297.1
Periodic amortization
Less: Components of amortization:
Amortization of Intangible Assets			(29.4)
Customer Relationships [Member]
Goodwill And Intangible Assets [Line Items]
Cost	675.1		675.1		671.7
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets			449.4
Less: Components of amortization:
Ending Balance, Finite Lived Intangible Assets	432.0		432.0
Finite-Lived Intangible Assets, Accumulated Amortization	243.1		243.1		222.3
Definite Lived Intangible Assets
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets			587.7
Less: Components of amortization:
Amortization of Intangible Assets	(14.4)	$ (15.1)	(29.4)	$ (30.5)
Effect of translation, Finite Lived intangible assets			2.2
Ending Balance, Finite Lived Intangible Assets	560.5		560.5
Definite Lived Intangible Assets | Periodic amortization
Less: Components of amortization:
Amortization of Intangible Assets			(29.4)
Trade Names [Member]
Goodwill And Intangible Assets [Line Items]
Cost	5.5		5.5		18.5
Intangible Assets, Including Goodwill [Roll Forward]
Beginning Balance, Finite Lived Intangible Assets			3.4
Less: Components of amortization:
Ending Balance, Finite Lived Intangible Assets	2.5		2.5
Finite-Lived Intangible Assets, Accumulated Amortization	$ 3.0		3.0		$ 15.1
Indefinite Lived Intangible Assets Excluding Goodwill [Member]
Less: Components of amortization:
Effect of translation, Indefinite Lived intangible assets			$ 4.7