SHARE-BASED COMPENSATION - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Options
Outstanding at beginning of period (in shares)	4,005,873
Granted (in shares)	717,000
Exercised (in shares)	(25,778)
Outstanding at end of period (in shares)	4,697,095		4,005,873
Exercisable at end of period (in shares)	2,626,106
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 10.67
Granted (in dollars per share)	29.26
Exercised (in dollars per share)	14.35
Outstanding at end of period (in dollars per share)	13.49		$ 10.67
Exercisable at end of period (in dollars per share)	$ 5.65
Aggregate intrinsic value and Weighted average remaining contractual life (years)
Outstanding (in dollars)	$ 63,082		$ 82,944
Exercisable (in dollars)	$ 53,554
Outstanding (in years)	7 years 6 months 4 days		7 years 4 months 2 days
Exercisable (in years)	6 years 2 months 27 days
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 16.94	$ 12.88