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           Stock-based Compensation (Details) (USD $) 
         | 
        3 Months Ended | |
|---|---|---|
| 
           Mar. 31, 2015 
         | 
        
           Mar. 31, 2014 
         | 
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| Stock-based compensation expense related to stock-based awards | ||
| Total stock-based compensation expense | $ 2,054,000us-gaap_AllocatedShareBasedCompensationExpense | $ 2,364,000us-gaap_AllocatedShareBasedCompensationExpense | 
| Research and development | ||
| Stock-based compensation expense related to stock-based awards | ||
| Total stock-based compensation expense | 
1,160,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember  | 
        
1,314,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember  | 
      
| General and administrative | ||
| Stock-based compensation expense related to stock-based awards | ||
| Total stock-based compensation expense | 
$ 894,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember  | 
        
$ 1,050,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember  | 
      
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- Definition 
          Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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