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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Costs and expenses:        
Total stock-based compensation expense $ 2,883 $ 2,129 $ 5,836 $ 4,036
Research and development [Member]        
Costs and expenses:        
Total stock-based compensation expense 1,734 1,163 3,425 2,236
General and administrative [Member]        
Costs and expenses:        
Total stock-based compensation expense $ 1,149 $ 966 $ 2,411 $ 1,800