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Derivative Financial Instruments - Summary of Gain (Loss) Recognized in Income on Derivatives (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Foreign exchange contracts | Other (expense) income, net      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain Recognized in Income on Derivatives $ (1.9) $ (3.1) $ (4.9)