Revenue (Details) - USD ($) $ in Thousands	3 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	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Jan. 01, 2018
Revenue
Accumulated Deficit	$ (1,013,316)				$ (962,438)				$ (1,013,316)	$ (962,438)
Accounts receivable	1,015				3,162				1,015	3,162
Contract assets	0				0				0	0
Contract liability	254				1,586				254	1,586
Revenue, contract liabilities									1,400
Revenues	887	$ 546	$ 715	$ 1,425	$ 1,764	$ 941	$ 2,763	$ 4,068	3,573	9,538	$ 12,743
ASC 606
Revenue
Accumulated Deficit													$ (1,300)
Deferred Revenue													(800)
Accounts receivable													$ 500
Product Development and Licensing Agreements
Revenue
Revenues									473	3,341	3,153
Contracts and Grants
Revenue
Revenues									3,100	6,197	9,590
Bristol-Myers Squibb Company | Product Development and Licensing Agreements
Revenue
Deferred Revenue	$ 0								0			$ 5,000
Reimbursement of external costs (as percent)												50.00%
Revenues									200	3,300	2,800
IAVI & Frontier | Contracts and Grants
Revenue
Revenues									300	3,000	6,600
Rockefeller | Contracts and Grants
Revenue
Revenues									$ 2,600	$ 3,200	$ 2,200