INCOME TAXES - Summary of Changes in Deferred Tax Asset Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets [Roll Forward]
Balance, January 1,	$ 164,865	$ 148,310	$ 136,017
Increase in deferred tax assets	27,183	16,555	12,293
Remeasurement in deferred tax assets	(59,336)	0	0
Balance, December 31,	$ 132,712	$ 164,865	$ 148,310