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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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