Purchased intangible assets - Components (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Assets	$ 53,413	$ 54,418
Accumulated Amortization	(50,395)	(50,124)
Net Assets	3,018	4,294
Technology and patents
Finite-Lived Intangible Assets [Line Items]
Gross Assets	37,029	37,684
Accumulated Amortization	(34,995)	(34,923)
Net Assets	2,034	2,761
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Assets	15,146	15,425
Accumulated Amortization	(15,026)	(14,835)
Net Assets	120	590
Leasehold interest
Finite-Lived Intangible Assets [Line Items]
Gross Assets	1,238	1,309
Accumulated Amortization	(374)	(366)
Net Assets	$ 864	$ 943