INCOME TAXES - Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	(0.40%)	(0.10%)	(0.20%)
Foreign rate differential	(14.90%)	(2.40%)	(1.80%)
Nondeductible expenses	(1.00%)	(0.90%)	(1.10%)
Credits	41.50%	8.50%	8.20%
Other items	(1.20%)	0.20%	0.10%
Change in valuation allowance	(119.50%)	(37.40%)	(38.60%)
Impact of U.S. tax reform	(209.10%)	0.00%	0.00%
Share-based compensation	243.50%	(2.40%)	(2.20%)
Change in uncertain tax positions	(2.40%)	(0.60%)	(0.50%)
Termination of warrant	29.30%	0.00%	0.00%
Total	(0.20%)	(1.10%)	(2.10%)