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, 2018	Sep. 30, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 41,403	$ 31,817
Unrealized Losses	(51)	(15)
Fair Value	41,352	31,802
Commercial paper and corporate bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	41,403	31,817
Unrealized Losses	(51)	(15)
Fair Value	$ 41,352	$ 31,802