Condensed Statements of Comprehensive Loss - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (4,102,729)	$ (5,840,902)	$ (9,482,676)	$ (11,657,176)
Other comprehensive income (loss), net of tax effect:
Unrealized gain (loss) on short-term investments	(188)	736	(11,339)	1,090
Comprehensive loss	$ (4,102,917)	$ (5,840,166)	$ (9,494,015)	$ (11,656,086)