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Investments - Summary of Investments by Type of Security (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Sep. 30, 2021
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | $ 194,333 | $ 197,165
Gross Unrealized Gains | 1 | 12
Gross Unrealized Losses | (326) | (69)
Fair Market Value | 194,008 | 197,108
Short-term investments | U.S. Treasury |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 7,711 | 4,222
Gross Unrealized Gains | 0 | 1
Gross Unrealized Losses | (11) | 0
Fair Market Value | 7,700 | 4,223
Short-term investments | Asset-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 4,514 | 4,812
Gross Unrealized Gains | 0 | 1
Gross Unrealized Losses | (2) | (2)
Fair Market Value | 4,512 | 4,811
Short-term investments | Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 125,632 | 140,042
Gross Unrealized Gains | 1 | 6
Gross Unrealized Losses | (92) | (25)
Fair Market Value | 125,541 | 140,023
Long-term investments | U.S. Treasury |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 7,497 | 6,996
Gross Unrealized Gains | 0 | 1
Gross Unrealized Losses | (29) | (2)
Fair Market Value | 7,468 | 6,995
Long-term investments | Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 46,081 | 38,184
Gross Unrealized Gains | 0 | 3
Gross Unrealized Losses | (181) | (39)
Fair Market Value | 45,900 | 38,148
Long-term investments | Foreign government and agency securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 2,898 | 2,909
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (11) | (1)
Fair Market Value | $ 2,887 | $ 2,908

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