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Investments (Tables) | 12 Months Ended
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Sep. 30, 2021
Held To Maturity Securities Classified [Abstract] |
Summary of Short-term, Long-term Investments and Marketable Securities | The following tables summarize the Company’s short-term, long-term investments and marketable securities as of September 30, 2021 and September 30, 2020 by measurement category.
Held to Maturity |  | As of September 30, 2021 |  |  |  |  |
|  | (in thousands) |  |  |  |  |
|  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |  |  |  |  |
Commercial notes (due within one year) |  | $ | 56,627 |  |  | $ | 803 |  |  | $ | - |  |  | $ | 57,430 |  |  |  |  |
Commercial notes (due within one through three years) |  | $ | 195,595 |  |  | $ | 1,151 |  |  | $ | (103 | ) |  | $ | 196,643 |  |  |  |  |
Certificate of deposit (due within two years) |  | $ | 50,000 |  |  | $ | - |  |  | $ | - |  |  | $ | 50,000 |  |  |  |  |
Total |  | $ | 302,222 |  |  | $ | 1,954 |  |  | $ | (103 | ) |  | $ | 304,073 |  |  |  |  |
|  | As of September 30, 2020 |  |  |  |  |
|  | (in thousands) |  |  |  |  |
|  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |  |  |  |  |
Commercial notes (due within one year) |  | $ | 86,890 |  |  | $ | 1,590 |  |  | $ | - |  |  | $ | 88,480 |  |  |  |  |
Commercial notes (due within one through three years) |  | $ | 137,487 |  |  | $ | 4,574 |  |  | $ | (79 | ) |  | $ | 141,982 |  |  |  |  |
Total |  | $ | 224,377 |  |  | $ | 6,164 |  |  | $ | (79 | ) |  | $ | 230,462 |  |  |  |  |
Fair Value |  | As of September 30, 2021 |  |  |  |  |
|  | (in thousands) |
|  | Cost |  |  | Realized Gains/(Losses) |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
Marketable securities |  | $ | 125,000 |  |  | $ | 2,481 |  |  | $ | 135 |  |  | $ | (888 | ) |  | $ | 126,728 |
Total |  | $ | 125,000 |  |  | $ | 2,481 |  |  | $ | 135 |  |  | $ | (888 | ) |  | $ | 126,728 |
Fair Value |  | As of September 30, 2020 |  |  |  |  |
|  | (in thousands) |
|  | Cost |  |  | Realized Gains/(Losses) |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
Marketable securities |  | $ | 85,000 |  |  | $ | 95 |  |  | $ | - |  |  | $ | (75 | ) |  | $ | 85,020 |
Total |  | $ | 85,000 |  |  | $ | 95 |  |  | $ | - |  |  | $ | (75 | ) |  | $ | 85,020 |

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