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STOCKHOLDERS' EQUITY - EQUITY PLANS - SCHEDULE OF AGGREGATE OPTION AND AWARD ACTIVITY (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Shares Available For Grant |  |  |
Balance at the beginning of the period (in shares) | 6,407,355 |  |
Additional shares authorized (in shares) | 7,000,000 |  |
Options granted (in shares) | (6,531,931) |  |
Awards granted (in shares) | (17,986) |  |
Options cancelled/forfeited/expired (in shares) | 400,309 |  |
Balance at the end of the period (in shares) | 7,257,747 | 6,407,355 |
Number of Shares - Options Outstanding |  |  |
Balance at the beginning of the period (in shares) | 37,614,013 |  |
Options granted (in shares) | 6,531,931 |  |
Options exercised (in shares) | (21,944) |  |
Options cancelled/forfeited/expired (in shares) | (460,309) |  |
Balance at the end of the period (in shares) | 43,663,691 | 37,614,013 |
Options exercisable at the end of the period (in shares) | 25,721,508 | 19,915,713 | 16,464,746
Options fully vested and expected to vest at the end of the period (in shares) | 42,621,956 |  |
Weighted Average Exercise Price Per Share - Options Outstanding |  |  |
Balance at the beginning of the period (in dollars per share) | $ 2.26 |  |
Options granted (in dollars per share) | 1.37 |  |
Options exercised (in dollars per share) | 1.16 |  |
Options cancelled/forfeited/expired (in dollars per share) | 3.48 |  |
Balance at the end of the period (in dollars per share) | 2.12 | $ 2.26 |
Options exercisable at the end of the period (in dollars per share) | 2.54 | $ 2.86 | $ 3.13
Options fully vested and expected to vest at the end of the period (in dollars per share) | $ 2.13 |  |
Weight Average Remaining Contractual Life (in years) - Options Outstanding |  |  |
Balance at the end of the period | 6 years 3 months 25 days |  |
Options exercisable at the end of the period | 4 years 10 months 24 days |  |
Options fully vested and expected to vest at the end of the period | 6 years 3 months 7 days |  |
Aggregate Intrinsic Value - Options Outstanding |  |  |
Balance at the end of the period (in dollars) | $ 5,624 |  |
Options exercisable at the end of the period (in dollars) | 2,247 |  |
Options fully vested and expected to vest at the end of the period (in dollars) | $ 5,437 |  |

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- Definition The number of shares under options that were cancelled or that expired during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References No definition available. + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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