Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	7,333	5,221	11,828	8,264
General and administrative	2,236	1,704	4,547	3,466
Total operating expenses	9,569	6,925	16,375	11,730
Loss from operations	(9,569)	(6,925)	(16,375)	(11,730)
Other income (expense):
Change in fair value of debt conversion feature liability	0	37	0	1,398
Amortization of debt discount	0	(146)	0	(404)
Amortization of financing costs	(30)	(30)	(60)	(60)
Interest income, net	1,925	392	3,839	573
Realized loss on investments	0	0	(2)	0
Total other income, net	1,895	253	3,777	1,507
Net loss	(7,674)	(6,672)	(12,598)	(10,223)
Other comprehensive loss, net of tax:
Unrealized gain (loss) on securities	288	(38)	662	(127)
Comprehensive loss	$ (7,386)	$ (6,710)	$ (11,936)	$ (10,350)
Basic and diluted net loss per common share	$ (0.11)	$ (0.13)	$ (0.18)	$ (0.21)
Weighted-average shares used to compute basic and diluted net loss per share	71,921,239	52,767,111	71,838,647	48,730,381