Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Effective income tax rate	(32.30%)	19.10%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized tax benefits, uncertain tax positions	$ 7.1		$ 6.8
Unrecognized tax benefits, accrued interest	0.4		$ 0.3
New Accounting Pronouncement, Early Adoption, Effect
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share-based compensation, tax benefit from compensation expense	$ 17.3