Balance sheet components - Purchased intangible assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Assets	$ 53,852	$ 53,413
Accumulated Amortization	(51,116)	(50,395)
Net Assets	2,736	3,018
Technology and patents
Finite-Lived Intangible Assets [Line Items]
Gross Assets	37,314	37,029
Accumulated Amortization	(35,457)	(34,995)
Net Assets	1,857	2,034
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Assets	15,269	15,146
Accumulated Amortization	(15,269)	(15,026)
Net Assets	0	120
Leasehold interest
Finite-Lived Intangible Assets [Line Items]
Gross Assets	1,269	1,238
Accumulated Amortization	(390)	(374)
Net Assets	$ 879	$ 864