Purchased intangible assets - Components (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Assets	$ 53,191	$ 53,413
Accumulated Amortization	(51,040)	(50,395)
Net Assets	2,151	3,018
Technology and patents
Finite-Lived Intangible Assets [Line Items]
Gross Assets	36,880	37,029
Accumulated Amortization	(35,555)	(34,995)
Net Assets	1,325	2,034
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Assets	15,089	15,146
Accumulated Amortization	(15,089)	(15,026)
Net Assets	0	120
Leasehold interest
Finite-Lived Intangible Assets [Line Items]
Gross Assets	1,222	1,238
Accumulated Amortization	(396)	(374)
Net Assets	$ 826	$ 864