Employee Benefit Plans and Stock-based compensation - Stock-based Compensation in Opex (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 28, 2018	Sep. 29, 2017	Sep. 28, 2018	Sep. 29, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 5,433	$ 3,720	$ 14,202	$ 11,107
Cost of Revenue [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	614	478	1,577	1,623
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	1,676	1,183	4,298	3,496
Selling, General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	3,143	2,059	8,327	5,988
Operating Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 4,819	$ 3,242	$ 12,625	$ 9,484