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MARKETABLE SECURITIES (Tables) | 12 Months Ended
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Dec. 31, 2020
Marketable Securities [Abstract] |
Marketable Securities [Table Text Block] | Amortized cost and estimated fair market value of marketable securities classified as available-for-sale (excluding cash equivalents) at December 31, 2020, were as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Gross Unrealized | ​ | Estimated Fair
(In thousands) |  | Cost |  | Gains |  | Losses |  | Market Value
Investments due in 3 months or less: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Commercial paper | ​ | $ | 43,660 | ​ | $ | — | ​ | $ | — | ​ | $ | 43,660
Corporate securities | ​ | ​ | 19,846 | ​ | ​ | 44 | ​ | ​ | — | ​ | ​ | 19,890
Total | ​ |  | 63,506 | ​ |  | 44 | ​ |  | — | ​ |  | 63,550
Investments due in 4-12 months: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Corporate securities | ​ |  | 125,922 | ​ |  | 846 | ​ |  | — | ​ |  | 126,768
Total | ​ |  | 125,922 | ​ |  | 846 | ​ |  | — | ​ |  | 126,768
Total marketable securities | ​ | $ | 189,428 | ​ | $ | 890 | ​ | $ | — | ​ | $ | 190,318
​ The Company did not have any investments due in twelve months or greater as of December 31, 2020. Accrued interest receivable was $0.8 million at December 31, 2020 and was recorded within prepaid expenses and other current assets on the condensed consolidated balance sheet. Amortized cost and estimated fair market value of marketable securities classified as available-for-sale (excluding cash equivalents) at December 31, 2019, were as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Gross Unrealized | ​ | Estimated Fair
(In thousands) |  | Cost |  | Gains |  | Losses |  | Market Value
Investments due in 3 months or less: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Corporate securities | ​ | $ | 15,934 | ​ | $ | 18 | ​ | $ | — | ​ | $ | 15,952
Total | ​ |  | 15,934 | ​ |  | 18 | ​ |  | — | ​ |  | 15,952
Investments due in 4-12 months: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Corporate securities | ​ |  | 71,223 | ​ |  | 269 | ​ |  | — | ​ |  | 71,492
Total | ​ |  | 71,223 | ​ |  | 269 | ​ |  | — | ​ |  | 71,492
Investments due in 12 months or greater: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Corporate securities | ​ |  | 144,658 | ​ |  | 302 | ​ |  | (6) | ​ |  | 144,954
Total | ​ |  | 144,658 | ​ |  | 302 | ​ |  | (6) | ​ |  | 144,954
Total marketable securities | ​ | $ | 231,815 | ​ | $ | 589 | ​ | $ | (6) | ​ | $ | 232,398
Marketable Securities, Available-for-sale, Unrealized Loss Position, Fair Value [Table Text Block] | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less Than 12 Months | ​ | 12 Months or Longer | ​ | Total
​ |  | Estimated |  | Gross |  | Estimated |  | Gross |  | Estimated |  | Gross
​ | ​ | Fair Market | ​ | Unrealized | ​ | Fair Market | ​ | Unrealized | ​ | Fair Market | ​ | Unrealized
(In thousands) | ​ | Value | ​ | Losses | ​ | Value | ​ | Losses | ​ | Value | ​ | Losses
December 31, 2019 | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Corporate securities | ​ | $ | 13,069 | ​ | $ | (6) | ​ | $ | — | ​ | $ | — | ​ | $ | 13,069 | ​ | $ | (6)
Total marketable securities | ​ | $ | 13,069 | ​ | $ | (6) | ​ | $ | — | ​ | $ | — | ​ | $ | 13,069 | ​ | $ | (6)

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- Definition Tabular disclosure of fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455 + Details
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- Definition Tabular disclosure of marketable securities. This may consist of investments in certain debt and equity securities, short-term investments and other assets. + References No definition available. + Details
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