Balance sheet components - Amortization expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Purchased intangible assets
Cost of goods sold	$ 853	$ 836	$ 2,542	$ 2,512
Operating expenses	462	447	1,375	1,344
Total	$ 1,315	$ 1,283	$ 3,917	$ 3,856