Fair Value Measurement of Company's Short-term Investments and Derivative Instruments (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 36,461	$ 36,137
Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Estimated Fair Value	23,711	22,798
Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Estimated Fair Value	12,750	13,339
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	36,461	36,137
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	23,711	22,798
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	$ 12,750	$ 13,339