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Income Taxes - Aggregate Changes in Balance of Company's Gross Unrecognized Tax Benefits (Parenthetical) (Detail) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Income Tax Disclosure [Abstract]      
Settlements with tax authorities $ 37,672 $ 3,880 $ 9,450