Balance sheet components - Purchased intangible assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Assets	$ 53,460	$ 53,413
Accumulated Amortization	(50,928)	(50,395)
Net Assets	2,532	3,018
Technology and patents
Finite-Lived Intangible Assets [Line Items]
Gross Assets	37,048	37,029
Accumulated Amortization	(35,368)	(34,995)
Net Assets	1,680	2,034
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Assets	15,172	15,146
Accumulated Amortization	(15,172)	(15,026)
Net Assets	0	120
Leasehold interest
Finite-Lived Intangible Assets [Line Items]
Gross Assets	1,240	1,238
Accumulated Amortization	(388)	(374)
Net Assets	$ 852	$ 864