Annual report pursuant to Section 13 and 15(d)

STOCK BASED COMPENSATION (Details)

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STOCK BASED COMPENSATION (Details) (USD $)
1 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2013
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Dec. 31, 2013
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Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2013
Research and development
Dec. 31, 2012
Research and development
Dec. 31, 2011
Research and development
Dec. 31, 2013
General and administrative
Dec. 31, 2012
General and administrative
Dec. 31, 2011
General and administrative
Sep. 30, 2013
Restructuring charges
Dec. 31, 2013
Restructuring charges
Stock-based compensation expense related to stock-based awards                        
Total stock-based compensation expense   $ 7,166,000 $ 12,617,000 $ 13,168,000 $ 3,930,000 $ 7,050,000 $ 9,277,000 $ 2,997,000 $ 5,567,000 $ 3,891,000 $ 239,000 $ 239,000
Reduction in workforce (as a percent) 18.00%                      
Number of positions eliminated 30                      
Period for exercise of vested options for terminated employees, excluding employees covered under restructuring   90 days                    
Number of terminated employees whose vesting period for unvested stock options was accelerated   1