Quarterly Financial Information (Unaudited) - Condensed Consolidated Statements of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Effect of Fourth Quarter Events [Line Items]
Total revenue	$ 1,379	$ 1,316	$ 1,101	$ 1,100	$ 4,036	$ 648	$ 649	$ 658	$ 2,201	$ 1,307	$ 3,517	$ 1,955	$ 4,896	$ 5,991
Operating expenses:
Research and development	5,558	8,598	6,189	4,827	4,837	5,697	6,176	6,335	11,016	12,511	19,614	18,208	25,172	23,045
General and administrative	1,817	2,290	3,311	2,994	2,712	3,295	2,620	2,880	6,305	5,500	8,595	8,795	10,412	11,507
Total operating expenses	7,375	10,888	9,500	7,821	7,549	8,992	8,796	9,215	17,321	18,011	28,209	27,003	35,584	34,552
Operating loss	(5,996)	(9,572)	(8,399)	(6,721)	(3,513)	(8,344)	(8,147)	(8,557)	(15,120)	(16,704)	(24,692)	(25,048)	(30,688)	(28,561)
Other income (expense), net:
Interest income	28	53	68	28	55	83	115	44	96	159	149	242	177	297
Interest expense	(1)	0	(1)	0	(262)	(262)	(261)	(262)	(1)	(523)	(1)	(785)	(2)	(1,047)
Other income, net	21	23	36	56	40	28	4	0	92	4	115	32	136	72
Total other income (expense), net	48	76	103	84	(167)	(151)	(142)	(218)	187	(360)	263	(511)	311	(678)
Net loss	(5,948)	(9,496)	(8,296)	(6,637)	(3,680)	(8,495)	(8,289)	(8,775)	(14,933)	(17,064)	(24,429)	(25,559)	(30,377)	(29,239)
Comprehensive loss	$ (5,948)	$ (9,496)	$ (8,296)	$ (6,637)	$ (3,680)	$ (8,495)	$ (8,289)	$ (8,775)	$ (14,933)	$ (17,064)	$ (24,429)	$ (25,559)	$ (30,377)	$ (29,239)
Net loss per share, basic and diluted (in USD per share)	$ (0.22)	$ (0.36)	$ (0.32)	$ (0.25)	$ (0.14)	$ (0.33)	$ (0.32)	$ (0.35)	$ (0.57)	$ (0.67)	$ (0.94)	$ (0.99)	$ (1.16)	$ (1.13)
Weighted-average common shares outstanding, basic and diluted (in shares)	26,685,133	26,189,454	26,069,734	26,066,064	26,056,504	26,046,666	26,039,169	25,026,890	26,067,909	25,535,827	26,108,870	25,707,978	26,254,119	25,795,721
License and collaboration revenue
Effect of Fourth Quarter Events [Line Items]
Total revenue	$ 1,176	$ 1,100	$ 1,101	$ 1,100	$ 4,036	$ 648	$ 649	$ 649	$ 2,201	$ 1,298	$ 3,301	$ 1,946	$ 4,477	$ 5,982
Government research contracts and grants revenue
Effect of Fourth Quarter Events [Line Items]
Total revenue	$ 203	$ 216	$ 0	$ 0					$ 0		$ 216		419	0
Research and development services revenue
Effect of Fourth Quarter Events [Line Items]
Total revenue					$ 0	$ 0	$ 0	$ 9		$ 9		$ 9	$ 0	$ 9