Revenue Recognition - Summary of Impact of Adoption of Accounting Standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Net loss	$ (11,948)	$ 289	$ (8,459)	$ (7,132)	$ (8,119)	$ (5,762)	$ (5,656)	$ (5,949)	$ (27,250)	$ (25,486)
Net loss per share - basic and diluted	$ (0.13)	$ 0	$ (0.10)	$ (0.09)	$ (0.10)	$ (0.08)	$ (0.08)	$ (0.09)	$ (0.31)	$ (0.35)
Costs and estimated earnings in excess of billings on uncompleted contracts	$ 987				$ 680				$ 987	$ 680
Other current assets	1,311				1,015				1,311	1,015
Billings on uncompleted contracts	0				5				0	5
Accumulated deficit	$ (546,116)				(518,866)				(546,116)	(518,866)
Product revenue
Revenue from Contract with Customer, Including Assessed Tax									0	2,300
Cost of Goods and Services Sold									5,468	4,359
License and royalty
Revenue from Contract with Customer, Including Assessed Tax									10,011	350
Contract
Revenue from Contract with Customer, Including Assessed Tax									7,596	6,984
Cost of Goods and Services Sold									$ 5,170	5,503
Topic 606 | Previously reported
Net loss										$ (16,382)
Net loss per share - basic and diluted										$ (0.23)
Costs and estimated earnings in excess of billings on uncompleted contracts					680					$ 680
Other current assets					945					945
Billings on uncompleted contracts					5					5
Deferred revenue - current					999					999
Deferred revenue - noncurrent					4,151					4,151
Accumulated deficit					(524,086)					(524,086)
Topic 606 | Previously reported | Product revenue
Revenue from Contract with Customer, Including Assessed Tax										2,300
Cost of Goods and Services Sold										4,359
Topic 606 | Previously reported | License and royalty
Revenue from Contract with Customer, Including Assessed Tax										1,568
Topic 606 | Previously reported | Contract
Revenue from Contract with Customer, Including Assessed Tax										7,023
Cost of Goods and Services Sold										5,517
Topic 606 | Adjustment
Net loss										$ (985)
Net loss per share - basic and diluted										$ (0.02)
Costs and estimated earnings in excess of billings on uncompleted contracts					0					$ 0
Other current assets					70					70
Billings on uncompleted contracts					0					0
Deferred revenue - current					(999)					(999)
Deferred revenue - noncurrent					(4,151)					(4,151)
Accumulated deficit					$ 5,220					5,220
Topic 606 | Adjustment | Product revenue
Revenue from Contract with Customer, Including Assessed Tax										0
Cost of Goods and Services Sold										0
Topic 606 | Adjustment | License and royalty
Revenue from Contract with Customer, Including Assessed Tax										(1,218)
Topic 606 | Adjustment | Contract
Revenue from Contract with Customer, Including Assessed Tax										(39)
Cost of Goods and Services Sold										$ (14)