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, 2019	Sep. 30, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 24,918	$ 41,403
Unrealized Gains	13
Unrealized Losses		(51)
Fair Value	24,931	41,352
Commercial paper and corporate bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	24,918	41,403
Unrealized Gains	13
Unrealized Losses		(51)
Fair Value	$ 24,931	$ 41,352