Goodwill and Intangibles, net - Summary of Definite Lived Intangible Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Finite-Lived Intangible Assets [Line Items]
Document Period End Date	Sep. 30, 2017
Cost	$ 1,423.9	$ 1,387.7
Accumulated Amortization	(548.7)	(488.7)
Net	875.2	899.0
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	1,007.6	984.8
Accumulated Amortization	(360.7)	(302.9)
Net	646.9	681.9
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	165.8	165.7
Accumulated Amortization	(104.6)	(89.1)
Net	61.2	76.6
Technology-Based Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	250.5	237.2
Accumulated Amortization	(83.4)	(96.7)
Net	$ 167.1	$ 140.5