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Marketable Securities | 6 Months Ended
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Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract] |
Marketable Securities | 4. Marketable Securities The Company invests in marketable securities that are classified as available-for-sale and records them at fair value in the accompanying unaudited Consolidated Balance Sheets. Marketable securities reported as current assets represent investments that mature within one year from the balance sheet date. Long-term marketable securities represent investments with maturity dates greater than one year from the balance sheet date. Unrealized gains and losses are excluded from earnings and reported as a separate component of “Accumulated other comprehensive income (loss)” in the accompanying unaudited Consolidated Balance Sheets until the security is sold or matures. Gains or losses realized from sales of marketable securities are computed based on the specific identification method and recognized as a component of “Other income (expenses)” in the accompanying unaudited Consolidated Statements of Operations. The Company had sales and maturities of marketable securities of $121.0 million and $728.2 million in the three and six months ended March 31, 2023, respectively. There were insignificant sales and maturities of marketable securities in each of the three and six months ended March 31, 2022, respectively. There were insignificant realized gain or loss in each of the three and six months ended March 31, 2023 and 2022. The following is a summary of the amortized cost and the fair value, including accrued interest receivable as well as unrealized gains (losses) on the short-term and long-term marketable securities as of March 31, 2023 and September 30, 2022 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​
​ | ​ | Cost | ​ | Losses | ​ | Gains | ​ | Fair Value
March 31, 2023: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 435,258 | ​ | $ | (3,672) | ​ | $ | 31 |  | $ | 431,617
Bank certificates of deposits | ​ | ​ | 10,355 | ​ | ​ | (187) | ​ | ​ | 1 | ​ | ​ | 10,169
Corporate securities | ​ | ​ | 341,868 | ​ | ​ | (5,370) | ​ | ​ | — | ​ | ​ | 336,498
Municipal securities |  | ​ | 1,544 | ​ | ​ | (1) | ​ | ​ | — |  | ​ | 1,543
​ | ​ | $ | 789,025 | ​ | $ | (9,230) | ​ | $ | 32 | ​ | $ | 779,827
September 30, 2022: | ​ | ​ |  | ​ |  |  | ​ |  |  | ​ |  |
U.S. Treasury securities and obligations of U.S. government agencies | ​ | $ | 804,774 |  | $ | (6,163) |  | $ | 21 | ​ | $ | 798,632
Bank certificates of deposits | ​ | ​ | 8,335 | ​ | ​ | (158) | ​ | ​ | 1 | ​ | ​ | 8,178
Corporate securities | ​ | ​ | 406,270 | ​ | ​ | (8,113) | ​ | ​ | — | ​ | ​ | 398,157
Municipal securities | ​ |  | 59,043 | ​ | ​ | (226) | ​ | ​ | — |  | ​ | 58,817
​ | ​ | $ | 1,278,422 | ​ | $ | (14,660) | ​ | $ | 22 | ​ | $ | 1,263,784
​ The fair values of the marketable securities by contractual maturities at March 31, 2023 are presented below (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Amortized | ​ | ​ | ​
​ |  | ​ | ​ | Cost | ​ | ​ | Fair Value
Due in one year or less | ​ | ​ | ​ | $ | 515,364 | ​ | $ | 513,652
Due after one year through five years | ​ | ​ | ​ |  | 270,591 | ​ |  | 263,105
Due after five years through ten years | ​ | ​ | ​ | ​ | — | ​ | ​ | —
Due after ten years | ​ | ​ | ​ |  | 3,070 | ​ |  | 3,070
Total marketable securities | ​ | ​ | ​ | $ | 789,025 | ​ | $ | 779,827
​ Expected maturities could differ from contractual maturities because the security issuers may have the right to prepay obligations without prepayment penalties. The Company reviews the marketable securities for impairment at each reporting period to determine if any of the securities have experienced an other-than-temporary decline in fair value. The Company considers factors, such as the length of time and extent to which the market value has been less than the cost, the financial condition and near-term prospects of the issuer, the Company’s intent to sell, or whether it is more likely than not it will be required to sell the investment before recovery of its amortized cost basis. If the Company believes that an other-than-temporary decline in fair value has occurred, it writes down the investment to its fair value and recognizes the credit loss in earnings and the non-credit loss in accumulated other comprehensive income or loss. Unrealized losses from fixed-income securities are primarily attributable to changes in interest rates. Management does not believe any unrealized losses represent impairments based on our evaluation of the available evidence.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI https://asc.fasb.org/subtopic&trid=2209399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI https://asc.fasb.org/subtopic&trid=2324412 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI https://asc.fasb.org/topic&trid=2196928 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI https://asc.fasb.org/subtopic&trid=2176304 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher SEC -Name Regulation S-K (SK) -Number 229 -Section 1403 -Paragraph (b) + Details
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