Revenue Recognition - Summary of Impact of Adoption of Accounting Standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Product revenue	$ 0	$ 45	$ 0	$ 45
Royalty revenue	0	106	11	203
Contract revenue	2,014	1,188	4,191	1,659
Cost of product revenue	326	181	564	394
Cost of contract revenue	1,355	812	2,990	1,215
Net loss	$ (8,459)	$ (5,656)	$ (15,591)	$ (11,605)
Net loss per share - basic and diluted	$ (0.10)	$ (0.08)	$ 0.19	$ (0.17)
Costs and estimated earnings in excess of billings on uncompleted contracts	$ 0		$ 0		$ 680
Other current assets	1,221		1,221		1,015
Billings on uncompleted contracts	446		446		5
Deferred revenue - current	5,000		5,000		0
Deferred revenue - noncurrent					0
Accumulated deficit	$ (534,457)		$ (534,457)		(518,866)
Topic 606 | Previously reported
Product revenue		$ 0		$ 0
Royalty revenue		346		881
Contract revenue		1,107		1,364
Cost of product revenue		135		348
Cost of contract revenue		810		1,135
Net loss		$ (5,494)		$ (11,141)
Net loss per share - basic and diluted		$ (0.08)		$ (0.16)
Costs and estimated earnings in excess of billings on uncompleted contracts					680
Other current assets					945
Billings on uncompleted contracts					5
Deferred revenue - current					999
Deferred revenue - noncurrent					4,151
Accumulated deficit					(524,086)
Topic 606 | Adjustment
Product revenue		$ 45		$ 45
Royalty revenue		(240)		(678)
Contract revenue		81		295
Cost of product revenue		46		46
Cost of contract revenue		2		80
Net loss		$ (162)		$ (464)
Net loss per share - basic and diluted		$ 0		$ (0.01)
Costs and estimated earnings in excess of billings on uncompleted contracts					0
Other current assets					70
Billings on uncompleted contracts					0
Deferred revenue - current					(999)
Deferred revenue - noncurrent					(4,151)
Accumulated deficit					$ 5,220