INCOME TAXES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other information
Excess tax benefit arising from stock option deductions credited to additional paid in capital	$ 45,500	$ 39,800
Deferred tax assets:
Operating loss carryforwards	189,007	140,471
Tax credit carryforwards	17,947	16,915
Deferred revenue		11
Other temporary differences	8,146	4,543
Tax assets before valuation allowance	215,100	161,940
Less-Valuation allowance	(215,100)	(161,940)
Valuation allowance
Change in valuation allowance	$ 53,200	$ 37,400
Differences between the effective income tax rate and the statutory tax rate
U.S. Federal statutory rate (as a percent)	35.00%	34.00%	34.00%
State taxes (as a percent)	2.10%	5.50%	4.80%
Federal and state tax rate changes (as a percent)	(1.70%)
Research and development tax credit (as a percent)	0.90%	(1.10%)	16.90%
Stock-based compensation expense (as a percent)	(0.60%)	(0.50%)	(1.10%)
Other adjustments (as a percent)	(0.90%)	(0.80%)	(0.30%)
Valuation allowance (as a percent)	(34.80%)	(37.10%)	(54.30%)
Effective tax rate (as a percent)	0.00%	0.00%	0.00%
Unrecognized tax benefits	$ 0	$ 0	$ 0
Accrued interest or penalties	0	0	0
Recognized interest or penalties	0	$ 0	$ 0
Internal Revenue Service
Operating loss carryforwards and tax credit carryforwards
Operating loss carryforwards	565,900
Internal Revenue Service | Research
Operating loss carryforwards and tax credit carryforwards
Tax credit carryforwards	7,500
State And Local Jurisdiction
Operating loss carryforwards and tax credit carryforwards
Operating loss carryforwards	208,900
State And Local Jurisdiction | Research
Operating loss carryforwards and tax credit carryforwards
Tax credit carryforwards	$ 16,000