SHARE-BASED COMPENSATION - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Options
Outstanding at beginning of period (in shares)			4,005,873
Granted (in shares)	155,000	0	1,196,000
Exercised (in shares)			(83,694)
Forfeited, shares (in shares)			(89,667)
Outstanding at end of period (in shares)	5,028,512		5,028,512		4,005,873
Exercisable at end of period (in shares)	2,900,840		2,900,840
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)			$ 10.67
Granted (in dollars per share)			35.98
Exercised (in dollars per share)			6.99
Forfeited (in dollars per share)			25.00
Outstanding at end of period (in dollars per share)	$ 16.50		16.50		$ 10.67
Exercisable at end of period (in dollars per share)	$ 6.81		$ 6.81
Aggregate intrinsic value and Weighted average remaining contractual life (years)
Outstanding (in dollars)	$ 113,742		$ 113,742		$ 82,944
Exercisable (in dollars)	$ 91,617		$ 91,617
Outstanding (in years)			7 years 3 months 3 days		7 years 4 months 2 days
Exercisable (in years)			5 years 11 months 19 days
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 23.72		$ 20.72	$ 12.34