Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Detail) - Assets Measured at Fair Value on a Recurring Basis [Member] - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	$ 52,998	$ 48,947
Available-for-sale securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	21,863	30,313
Cash equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	31,135	18,634
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	52,998	48,947
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	21,863	30,313
Significant Other Observable Inputs (Level 2) [Member] | Cash equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	$ 31,135	$ 18,634