Employee Benefit Plans and Stock-based compensation - Stock-based Compensation in Opex (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 25, 2020	Sep. 27, 2019	Sep. 25, 2020	Sep. 27, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	$ 3,930	$ 4,096	$ 13,737	$ 8,719
Cost of Revenue [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	284	410	1,366	830
Research and Development Expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	1,110	1,120	3,720	2,318
Selling, General and Administrative Expenses [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	2,536	2,566	8,651	5,571
Operating Expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	$ 3,646	$ 3,686	$ 12,371	$ 7,889