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Investments - Summary of Investments by Type of Security (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Sep. 30, 2017
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | $ 20,171 | $ 34,075
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (86) | (16)
Fair Market Value | 20,085 | 34,059
U.S. Treasury | Short-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 8,654 | 3,897
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (30) | (3)
Fair Market Value | 8,624 | 3,894
Corporate debt securities | Short-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 10,572 | 26,393
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (50) | (8)
Fair Market Value | 10,522 | 26,385
Corporate debt securities | Long-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 945 | 3,785
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (6) | (5)
Fair Market Value | $ 939 | $ 3,780

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