Quarterly Financial Information (Unaudited) - Condensed Consolidated Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 13,711	$ 15,549	$ 22,438	$ 6,077	$ 8,194	$ 12,173	$ 20,984	$ 28,100
Restricted cash	0	5,651	8,286	0
Contracts and grants receivable	419	216	0	0	0
Deferred offering costs	49	49	49	49	49	49	49	49
Prepaid expenses and other current assets	1,545	982	1,237	1,062	1,107	584	576	889
Total current assets	15,724	22,447	32,010	7,188	9,350	12,806	21,609	29,038
Restricted cash	540	1,345	2,060	539	539	539	539	539
Intangible assets	75	75	75	75	75	75	75	75
Other assets	419	444	473	501	530	146	161	176
Property and equipment, net	10,506	10,569	11,054	11,657	15,868	16,129	16,327	16,474
Right-of-use lease assets	1,833	1,837	1,837	1,833	0
Total assets	29,097	36,717	47,509	21,793	26,362	29,695	38,711	46,302
Current liabilities:
Accounts payable	1,602	2,495	957	1,510	1,250	386	1,056	828
Accrued compensation	437	1,507	1,412	2,082	1,467	1,689	1,380	977
Accrued outside research and development services	1,013	1,382	789	817	563	1,571	1,408	1,258
Accrued legal and professional fees	616	168	295	258	498	305	267	458
Other accrued expenses	553	878	633	516	871	1,247	1,443	1,194
Deferred revenue, current portion	4,428	4,401	4,401	4,401	4,401	2,595	2,595	2,638
Research and development service obligation liability, current portion	3,088	5,651	8,286	0	0
Lease liabilities, current portion	1,162	1,156	1,151	1,139	11	11	11	11
Total current liabilities	12,899	17,638	17,924	10,723	9,061	7,804	8,160	7,364
Deferred revenue, net of current portion	7,076	3,725	4,825	5,926	2,566	4,000	4,648	5,294
Lease liabilities, net of current portion	5,100	5,249	5,395	5,544	10	13	16	19
Research and development service obligation liability, net of current portion	727	806	1,521	0	0
Research and development funding arrangement liability, related party	25,000	25,000	25,000	0	0
Other long-term liabilities	578	1,634	1,630	335	289	103	0	0
Facility financing obligation	0				7,998	7,998	7,998	7,998
Total liabilities	51,380	54,052	56,295	22,528	19,924	19,918	20,822	20,675
Commitments and contingencies
Stockholders' equity (deficit)
Common stock $0.0001 par value	3	3	3	3	3	3	3	3
Additional paid-in-capital	197,853	196,853	195,906	195,661	195,483	195,142	194,759	194,208
Treasury stock at cost	(155)	(155)	(155)	(155)	(155)	(155)	(155)	(155)
Accumulated deficit	(219,984)	(214,036)	(204,540)	(196,244)	(188,893)	(185,213)	(176,718)	(168,429)
Total stockholders' (deficit) equity	(22,283)	(17,335)	(8,786)	(735)	6,438	9,777	17,889	25,627	$ (1,716)
Total liabilities and stockholders’ (deficit) equity	$ 29,097	$ 36,717	$ 47,509	$ 21,793	$ 26,362	$ 29,695	$ 38,711	$ 46,302
Common stock, par value (in USD per share)	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001