Purchased intangible assets - Amortization Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Cost of goods sold	$ 737	$ 756	$ 869
Operating expenses	119	475	472
Total	$ 856	$ 1,231	$ 1,341