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INCOME TAXES - DEFERRED TAXES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant components of deferred tax assets
Net operating loss carryforwards	$ 204,600,000	$ 192,100,000
Research credits	38,400,000	35,500,000
Capitalized research and development	5,900,000	2,500,000
Stock-based compensation	7,700,000	6,400,000
Operating lease liabilities	700,000
Other	1,200,000	600,000
Total deferred tax assets	258,500,000	237,100,000
Less: valuation allowance	(257,900,000)	(237,100,000)
Net deferred tax assets	600,000
Operating leases, right-of-use assets	(600,000)
Total deferred tax liabilities	(600,000)
Valuation allowance
Increase (decrease) in valuation allowance	$ 20,800,000	$ 4,300,000	$ (89,400,000)

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