Income Taxes (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Effective income tax rate	28.60%	30.40%	9.20%	27.20%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized tax benefits, uncertain tax positions	$ 7.8		$ 7.8		$ 6.8
Unrecognized tax benefits, accrued interest	0.5		0.5		$ 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	$ 2.7		$ 23.5