Fair Value of Company's Short-term Investments and Foreign Currency Forward Contracts (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Foreign Currency Forward Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net Derivatives		$ 5	$ (18)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 36,137	27,496	18,094
Fair Value, Measurements, Recurring | Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Estimated Fair Value	22,798	17,323	15,161
Fair Value, Measurements, Recurring | Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Estimated Fair Value	13,339	10,168	2,951
Fair Value, Measurements, Recurring | Foreign Currency Forward Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net Derivatives		5	(18)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	36,137	27,491	18,112
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Estimated Fair Value	22,798	17,323	15,161
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Estimated Fair Value	$ 13,339	10,168	2,951
Fair Value, Measurements, Recurring | Significant Other Observable (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total		5	(18)
Fair Value, Measurements, Recurring | Significant Other Observable (Level 2) | Foreign Currency Forward Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net Derivatives		$ 5	$ (18)