Annual report pursuant to Section 13 and 15(d)

STOCK-BASED COMPENSATION (Details)

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STOCK-BASED COMPENSATION (Details)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2013
item
Dec. 31, 2014
USD ($)
Sep. 30, 2013
USD ($)
item
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Stock-based compensation expense related to stock-based awards              
Total stock-based compensation expense     $ 1,500,000   $ 7,403,000 $ 9,787,000 $ 7,166,000
Reduction in workforce (as a percent)   18.00%          
Number of positions eliminated | item   30          
Period for exercise of vested options for terminated employees, excluding employees covered under restructuring 90 days 90 days          
Remaining contractual term for recently granted options 9 years            
Number of terminated employees whose vesting period for unvested stock options was accelerated | item       1      
Research and development              
Stock-based compensation expense related to stock-based awards              
Total stock-based compensation expense         4,100,000 4,674,000 3,930,000
General and administrative              
Stock-based compensation expense related to stock-based awards              
Total stock-based compensation expense         $ 3,303,000 $ 5,113,000 2,997,000
Restructuring charge              
Stock-based compensation expense related to stock-based awards              
Total stock-based compensation expense       $ 239,000     $ 239,000