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STOCK-BASED COMPENSATION - Summary of Restricted Stock Unit Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Aggregate Intrinsic Value      
Share price (in USD per share) $ 26.13    
Restricted stock units      
Number of Shares Outstanding      
Outstanding, beginning balance (in shares) 4,352 3,505 606
Granted (in shares) 2,112 3,152 5,418
Vested (in shares) (1,707) (1,399) (885)
Canceled (in shares) (494) (906) (1,634)
Outstanding, ending balance (in shares) 4,263 4,352 3,505
Number of shares outstanding, expected to vest (in shares) 4,263    
Weighted Average Fair Value per Share at Grant Date      
Outstanding, beginning balance (in usd per share) $ 3.52 $ 2.03 $ 9.33
Granted (in usd per share) 11.50 4.45 1.46
Vested (in usd per share) 3.87 2.75 3.81
Canceled (in usd per share) 4.81 2.17 1.90
Outstanding, ending balance (in usd per share) 7.19 $ 3.52 $ 2.03
Weighted-Average Fair Value per Share at Grant Date, Expected to vest (in usd per share) $ 7.19    
Weighted-Average Remaining Contractual Term      
Weighted- Average Remaining Contractual Term 1 year 3 months 18 days    
Expected to vest 1 year 3 months 18 days    
Aggregate Intrinsic Value      
Vested $ 27,156 $ 6,657 $ 932
Outstanding 111,387    
Aggregate intrinsic value, expected to vest $ 111,387