Condensed Consolidated Statements of Comprehensive Loss (unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (17,675,373)	$ (13,387,672)	$ (49,244,511)	$ (25,392,600)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	(103,505)	49,391	414,368	(37,472)
Comprehensive loss	$ (17,778,878)	$ (13,338,281)	$ (48,830,143)	$ (25,430,072)