SHARE-BASED COMPENSATION - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Options
Outstanding at beginning of period (in shares)			3,357,873
Granted (in shares)	0		356,000
Outstanding at end of period (in shares)	3,713,873		3,713,873		3,357,873
Exercisable at end of period (in shares)	2,243,931		2,243,931
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)			$ 7.74
Granted (in dollars per share)			21.50
Outstanding at end of period (in dollars per share)	$ 9.06		9.06		$ 7.74
Exercisable at end of period (in dollars per share)	$ 4.04		$ 4.04
Aggregate intrinsic value and Weighted average remaining contractual life (years)
Outstanding (in dollars)	$ 63,144		$ 63,144		$ 43,224
Exercisable (in dollars)	$ 49,408		$ 49,408
Outstanding (in years)			7 years 4 months 21 days		7 years 10 months 24 days
Exercisable (in years)			6 years 5 months 5 days
Weighted average grant-date fair value of stock options granted (in dollars per share)		$ 7.54	$ 12.34	$ 6.77