STOCK-BASED COMPENSATION - SUMMARY OF TIME-BASED STOCK OPTIONS (Details) - Time-based stock options $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Number of Shares Underlying Stock Options
Beginning outstanding balance (in shares) | shares	1,862
Granted (in shares) | shares	783
Exercised (in shares) | shares	(173)
Canceled or expired (in shares) | shares	(733)
Ending outstanding balance (in shares) | shares	1,739	1,862
Number of shares underlying stock options, Vested and expected to vest (in shares) | shares	1,739
Number of shares underlying stock options, Exercisable (in shares) | shares	927
Weighted Average Exercise Price Per Share
Beginning outstanding balance (in dollars per share) | $ / shares	$ 9.44
Granted (in dollars per share) | $ / shares	8.25
Exercised (in dollars per share) | $ / shares	7.16
Canceled or expired (in dollars per share) | $ / shares	9.78
Ending outstanding balance (in dollars per share) | $ / shares	8.98	$ 9.44
Weighted average exercise price, Vested and expected to vest (in dollars per share) | $ / shares	8.98
Weighted average exercise price, Exercisable (in dollars per share) | $ / shares	$ 9.57
Weighted average remaining contractual life, Outstanding	3 years 4 months 6 days	2 years 10 months 6 days
Weighted average remaining contractual life, Vested and expected to vest	3 years 4 months 6 days
Weighted average remaining contractual life, Exercisable	6 months 7 days
Aggregate intrinsic value, Outstanding | $	$ 128	$ 751
Aggregate intrinsic value, Vested and expected to vest | $	128
Aggregate intrinsic value, Exercisable | $	$ 10