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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Costs and expenses:        
Total stock-based compensation expense $ 2,129 $ 1,378 $ 4,036 $ 2,707
Research and development [Member]
        
Costs and expenses:        
Total stock-based compensation expense 1,163 723 2,236 1,408
General and administrative [Member]
        
Costs and expenses:        
Total stock-based compensation expense $ 966 $ 655 $ 1,800 $ 1,299