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Investments (Tables) | 6 Months Ended
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Mar. 31, 2021
Held To Maturity Securities Classified [Abstract] |
Summary of Short-term and Long-term Investments and Marketable Securities | The following tables summarize the Company’s short-term and long-term investments and marketable securities as of March 31, 2021 and September 30, 2020 by measurement category.
Held to Maturity |  | As of March 31, 2021 |
|  | (In thousands) |
|  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
Commercial notes (due within one year) |  | $ | 79,148 |  |  | $ | 887 |  |  | $ | - |  |  | $ | 80,035 |
Commercial notes (due within one through three years) |  | $ | 97,490 |  |  | $ | 2,655 |  |  | $ | (66 | ) |  | $ | 100,079 |
Total |  | $ | 176,638 |  |  | $ | 3,542 |  |  | $ | (66 | ) |  | $ | 180,114 |
|  | 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 |  |  | $ | 4,573 |  |  | $ | (79 | ) |  | $ | 141,981 |
Total |  | $ | 224,377 |  |  | $ | 6,163 |  |  | $ | (79 | ) |  | $ | 230,461 |
Fair Value |  | As of March 31, 2021 |
|  | (In thousands) |
|  | Cost |  |  | Realized Gains/(Losses) |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
Marketable securities |  | $ | 125,000 |  |  | $ | 1,079 |  |  | $ | 205 |  |  | $ | (491 | ) |  | $ | 125,793 |
Total |  | $ | 125,000 |  |  | $ | 1,079 |  |  | $ | 205 |  |  | $ | (491 | ) |  | $ | 125,793 |
|  | 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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