Purchased intangible assets - Amortization expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Cost of goods sold	$ 756	$ 869	$ 2,871
Operating expenses	475	472	1,609
Total	$ 1,231	$ 1,341	$ 4,480