Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Sep. 30, 2015
Schedule Of Available For Sale Securities [Line Items]
Short-term Investments, Amortized Cost	$ 36,192				$ 27,511	$ 18,114
Short-term Investments, Unrealized Gains						3
Short-term Investments, Unrealized Losses	(55)				(20)	(5)
Short-term Investments, Estimated Fair Value	36,137	$ 36,461	$ 28,862	$ 24,739	27,491	18,112
Corporate bonds
Schedule Of Available For Sale Securities [Line Items]
Short-term Investments, Amortized Cost	22,829				17,342	15,166
Short-term Investments, Unrealized Losses	(31)				(19)	(5)
Short-term Investments, Estimated Fair Value	22,798				17,323	15,161
Government bonds
Schedule Of Available For Sale Securities [Line Items]
Short-term Investments, Amortized Cost	13,363				10,169	2,948
Short-term Investments, Unrealized Gains						3
Short-term Investments, Unrealized Losses	(24)				(1)
Short-term Investments, Estimated Fair Value	$ 13,339				$ 10,168	$ 2,951