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Income Taxes (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Sep. 30, 2016
Income Tax Disclosure [Abstract]          
Effective income tax rate 24.30% 28.10% (1.90%) 24.30%  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unrecognized tax benefits, uncertain tax positions $ 7.5   $ 7.5   $ 6.8
Unrecognized tax benefits, accrued interest 0.4   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 $ 3.6   $ 20.9