CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Total revenue	$ 85,855	$ 32,500	$ 116,175	$ 57,500
Operating expenses:
Research and development	8,856,028	4,089,677	21,735,588	20,739,170
General and administrative	4,788,918	3,011,176	13,198,445	10,061,722
Acquired in-process research and development		16,493,132		16,493,132
Total operating expenses	13,644,946	23,593,985	34,934,033	47,294,024
Loss from operations	(13,559,091)	(23,561,485)	(34,817,858)	(47,236,524)
Other income (expense), net:
Interest income, net	959,092	264,932	2,092,397	734,065
Other income (expense), net	(7,526)	29,735	(49,353)	2,328
Net loss	$ (12,607,525)	$ (23,266,818)	$ (32,774,814)	$ (46,500,131)
Net loss per share -basic and diluted (in dollars per share)	$ (0.17)	$ (0.55)	$ (0.50)	$ (1.09)
Items included in other comprehensive income:
Unrealized gain on available-for-sale securities	$ 11,640	$ 30,303	$ 41,749	$ 36,636
Other comprehensive income	11,640	30,303	41,749	36,636
Comprehensive loss	$ (12,595,885)	$ (23,236,515)	$ (32,733,065)	$ (46,463,495)
Weighted-average number of common shares used in net loss per share calculation - basic and diluted (in shares)	72,043,113	42,636,567	65,648,006	42,525,693
Collaboration
Revenue:
Type of Revenue [Extensible List]	Collaboration [Member]		Collaboration [Member]