Condensed Statements of Comprehensive Loss - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (5,379,947)	$ (5,816,274)
Other comprehensive income (loss), net of tax effect:
Unrealized gain (loss) on short-term investments	(11,151)	354
Comprehensive loss	$ (5,391,098)	$ (5,815,920)