Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2014	Jan. 01, 2018	Dec. 31, 2017
Revenue
Accumulated Deficit	$ (945,792)		$ (945,792)				$ (812,517)
Accounts receivable	3,423		3,423				$ 1,880
Revenue recognized from contract liabilities	1,000		1,500
Revenues	2,763	$ 3,829	6,834	$ 5,363
ASU 2014-09
Revenue
Accumulated Deficit						$ (1,300)
Deferred revenue						(800)
Accounts receivable						$ 500
Product Development and Licensing Agreements
Revenue
Revenues	1,667	694	2,662	1,250
Contracts and Grants
Revenue
Revenues	1,096	3,135	4,172	4,113
Bristol-Myers Squibb Company | Product Development and Licensing Agreements
Revenue
Deferred revenue					$ 5,000
Reimbursement of external costs (as percent)					50.00%
Revenues	1,700		2,600
IAVI & Frontier | Contracts and Grants
Revenue
Revenues	400	2,300	2,700	2,300
Rockefeller | Contracts and Grants
Revenue
Revenues	$ 600	$ 300	$ 1,300	$ 1,100