SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - 2017 Warrants (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Warrant or Right [Line Items]
Increase in warrant fair value	$ 4,836	$ 3,587	$ (10,450)
2017 Warrants
Class of Warrant or Right [Line Items]
Fair value of warrants	25,300	20,500
Increase in warrant fair value	$ 4,800	$ 3,600