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	Jun. 30, 2021	Sep. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	$ 15,017	$ 48,947
Available-for-sale securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	9,791	30,313
Cash equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets measured at fair value	5,226	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	15,017	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	9,791	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	$ 5,226	$ 18,634