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Investments (Tables) | 3 Months Ended | 6 Months Ended
--- | --- | ---
Dec. 31, 2021 | Mar. 31, 2022
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, 2022 and September 30, 2021 by measurement category: |
|  | As of March 31, 2022 |  |  |
|  | (In thousands) |  |  |
|  | Cost Basis |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |  |  |
Classified as Cash Equivalents |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial notes (due within ninety days) |  | $ | 26,678 |  |  | $ | 1 |  |  | $ | (1 | ) |  | $ | 26,678 |  |  |
Classified as Held to Maturity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial notes (due within one year) |  | $ | 142,912 |  |  | $ | 225 |  |  | $ | (307 | ) |  | $ | 142,830 |  |  |
Commercial notes (due within one through three years) |  | $ | 201,590 |  |  | $ | - |  |  | $ | (5,864 | ) |  | $ | 195,726 |  |  |
Certificate of deposit (due within one year) |  | $ | 50,000 |  |  | $ | - |  |  | $ | - |  |  | $ | 50,000 |  |  |
Classified as Marketable Securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Marketable securities |  | $ | 128,805 |  |  | $ | - |  |  | $ | (6,230 | ) |  | $ | 122,575 |  |  |
Total |  | $ | 549,985 |  |  | $ | 226 |  |  | $ | (12,402 | ) |  | $ | 537,809 |  |  |
|  | As of September 30, 2021 |  |  |
|  | (In thousands) |  |  |
|  | Cost Basis |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |  |  |
Classified as Held to Maturity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 one through two years) |  | $ | 50,000 |  |  | $ | - |  |  | $ | - |  |  | $ | 50,000 |  |  |
Classified as Marketable Securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Marketable securities |  | $ | 127,481 |  |  | $ | - |  |  | $ | (753 | ) |  | $ | 126,728 |  |  |
Total |  | $ | 429,703 |  |  | $ | 1,954 |  |  | $ | (856 | ) |  | $ | 430,801 |  |  |

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