STOCK BASED COMPENSATION - Stock Options (Details) - 2009 plan $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Number of Shares
Outstanding at the beginning of the period (in shares) | shares	7,453,076
Granted (in shares) | shares	1,477,422
Exercised (in shares) | shares	(311,028)
Forfeited | shares	(45,897)
Outstanding at the end of the period (in shares) | shares	8,573,573	7,453,076
Vested and expected to vest (in shares) | shares	8,573,573
Options exercisable (in shares) | shares	4,656,466
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 7.46
Granted (in dollars per share) | $ / shares	3.88
Exercised (in dollars per share) | $ / shares	3.02
Forfeited (in dollars per share) | $ / shares	6.26
Outstanding at the end of the period (in dollars per share) | $ / shares	7.01	$ 7.46
Vested and expected to vest (in dollars per share ) | $ / shares	7.01
Options exercisable (in dollars per shares) | $ / shares	$ 7.39
Weighted Average Remaining Contractual Term
Weighted average period	7 years 8 months 12 days	7 years 8 months 12 days
Vested and expected to vest	7 years 8 months 12 days
Options exercisable	6 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period (in dollars) | $	$ 1,313
Outstanding at the end of the period (in dollars) | $	40,475	$ 1,313
Vested and expected to vest (in dollars) | $	40,475
Options exercisable (in dollars) | $	$ 21,417