Condensed Consolidated Statements of Comprehensive Loss (unaudited) - 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	$ (13,387,672)	$ (6,397,272)	$ (25,392,600)	$ (19,321,180)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	49,391	9,339	(37,472)	18,043
Comprehensive loss	$ (13,338,281)	$ (6,387,933)	$ (25,430,072)	$ (19,303,137)