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INCOME TAXES - DEFERRED TAXES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Significant components of deferred tax assets
Net operating loss carryforwards	$ 217,100,000	$ 204,600,000
Federal and state tax credits	42,800,000	38,400,000
Capitalized research and development	6,000,000	5,900,000
Stock-based compensation	9,400,000	7,700,000
Operating lease liabilities	1,200,000	700,000
Other	2,100,000	1,200,000
Total deferred tax assets	278,600,000	258,500,000
Less: valuation allowance	(277,200,000)	(257,900,000)
Net deferred tax assets	1,400,000	600,000
Operating leases, right-of-use assets	(1,400,000)	(600,000)
Total deferred tax liabilities	(1,400,000)	(600,000)
Valuation allowance
Increase (decrease) in valuation allowance	$ 19,300,000	$ 20,800,000

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