Stock Plans and Stock Based Compensation - Employee and Non-Employee Share Based Compensation Expense Allocation (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Total stock-based compensation expense	$ 3,875,246	$ 3,059,989	$ 3,002,241
Research and Development Expenses [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Total stock-based compensation expense	1,023,269	671,063	879,052
General and Administrative Expenses [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Total stock-based compensation expense	$ 2,851,977	$ 2,388,926	$ 2,123,189