Condensed Statements of Comprehensive Loss - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (4,843,408)	$ (5,380,846)	$ (14,326,084)	$ (17,038,022)
Other comprehensive income (loss), net of tax effect:
Unrealized gain (loss) on short-term investments	2,457	0	(8,882)	1,090
Comprehensive loss	$ (4,840,951)	$ (5,380,846)	$ (14,334,966)	$ (17,036,932)