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Nature of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization, property and equipment	$ 22.6	$ 23.0	$ 17.7
Foreign exchange gains (losses)	0.4	(1.1)	0.2
Advertising and promotion costs	$ 4.1	$ 3.1	$ 3.6
Minimum
Property, Plant and Equipment [Line Items]
Stock option awards vesting period	3 years
Maximum
Property, Plant and Equipment [Line Items]
Stock option awards vesting period	4 years
Internal-use Software \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	2 years
Internal-use Software \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	3 years