Fair Value Measurement - Summary of Company's Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Assets
Assets, Totals	$ 290,002	$ 138,107
Corporate bonds [Member]
Assets
Assets, Totals		8,530
Commercial Paper [Member]
Assets
Assets, Totals	94,840	28,361
US Government Treasury Bills [Member]
Assets
Assets, Totals	101,495	54,482
Cash [Member]
Assets
Assets, Totals	20,254	19,344
Money Market Funds [Member]
Assets
Assets, Totals	73,413	27,390
Fair Value, Inputs, Level 1 [Member]
Assets
Assets, Totals	195,162	101,216
Fair Value, Inputs, Level 1 [Member] | US Government Treasury Bills [Member]
Assets
Assets, Totals	101,495	54,482
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Assets
Assets, Totals	20,254	19,344
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets
Assets, Totals	73,413	27,390
Fair Value, Inputs, Level 2 [Member]
Assets
Assets, Totals	94,840	36,891
Fair Value, Inputs, Level 2 [Member] | Corporate bonds [Member]
Assets
Assets, Totals		8,530
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets
Assets, Totals	$ 94,840	$ 28,361