Summary of Significant Accounting Policies and Select Balance Sheet Information - Disclosure - Amortized Cost, Unrealized Holding Gains and (Losses) and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 31,817	$ 22,019
Unrealized Losses	(15)	(65)
Fair Value	31,802	21,954
Commercial paper and corporate bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	31,817	22,019
Unrealized Losses	(15)	(65)
Fair Value	$ 31,802	$ 21,954