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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)

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- Definition Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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