Stockholders' Equity - Stock-based Compensation Expense (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense (income)	$ 6,115,160	$ 1,754,974	$ 2,072,210
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense (income)	1,584,165	391,946	613,629
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense (income)	$ 4,530,995	$ 1,363,028	$ 1,458,581