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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense and effect on net loss $ 4,244,408 $ 2,034,898 $ 2,608,670
Research and Development Expense [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense and effect on net loss 1,873,848 608,840 1,219,308
General and Administrative Expense [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense and effect on net loss $ 2,370,560 $ 1,426,058 $ 1,389,362