SHARE-BASED COMPENSATION - Stock option expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-Based Compensation Awards
Options granted (in shares)	209,000	324,000	209,000	1,041,000
Stock options \| Employees and directors
Share-Based Compensation Awards
Stock-based compensation expenses	$ 4,752	$ 2,406	$ 9,380	$ 4,594
Stock options \| Employees and directors \| Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	1,753	537	3,444	1,026
Stock options \| Employees and directors \| General and administrative
Share-Based Compensation Awards
Stock-based compensation expenses	$ 2,999	$ 1,871	$ 5,936	$ 3,570

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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