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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Costs and expenses:        
Total stock-based compensation expense $ 1,391 $ 1,404 $ 4,098 $ 4,011
Research and development [Member]
        
Costs and expenses:        
Total stock-based compensation expense 732 774 2,140 2,195
General and administrative [Member]
        
Costs and expenses:        
Total stock-based compensation expense $ 659 $ 630 $ 1,958 $ 1,816