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Income Taxes - Summary of Changes to Unrecognized Tax Benefits Reserves Excluding Related Interest and Penalties (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Income Tax Contingency [Line Items]        
Unrecognized Tax Benefits $ 47.9 $ 16.1 $ 12.6 $ 13.8
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions 29.9 4.7 2.7  
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions 2.3 1.9 1.4  
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions 4.8 1.8 0.8  
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities 0.6 0.8 2.5  
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations $ 0.0 $ 0.3 $ 0.8