Acquisitions - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Goodwill (Note 12)	$ 2,280.2	$ 2,277.1
Estimated Income tax rate	21.00%	35.00%
Technology Assets [Member] | Minimum [Member]
Business Acquisition [Line Items]
Amortization period, intangible asset	6 years
Technology Assets [Member] | Maximum
Business Acquisition [Line Items]
Amortization period, intangible asset	18 years
Customer Relationships [Member] | Minimum [Member]
Business Acquisition [Line Items]
Amortization period, intangible asset	2 years
Customer Relationships [Member] | Maximum
Business Acquisition [Line Items]
Amortization period, intangible asset	20 years