Quarterly report pursuant to Section 13 or 15(d)

Common Stock - Summary of Share-Based Compensation Expense for Stock Options and Restricted Stock (Detail)

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Common Stock - Summary of Share-Based Compensation Expense for Stock Options and Restricted Stock (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Total stock-based compensation $ 712 $ 1,016 $ 1,228 $ 2,016
Research and development [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Total stock-based compensation 317 495 569 942
General and administrative [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Total stock-based compensation $ 395 $ 521 $ 659 $ 1,074