Preferred Stock, Common Stock and Stockholders' Equity - Summary of Recognized Non-Cash Stock-Based Compensation Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,819,431	$ 1,706,524
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	819,815	698,032
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 999,616	$ 1,008,492