Summary of Significant Accounting Policies and Select Balance Sheet Information - Amortized Cost, Unrealized Holding Gains and (Losses) and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 30,313	$ 24,918
Unrealized Gains	19	13
Unrealized Losses	(19)
Fair Value	30,313	24,931
Commercial paper and corporate bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	30,313	24,918
Unrealized Gains	19	13
Unrealized Losses	(19)
Fair Value	$ 30,313	$ 24,931