Employee Benefit Plans and Stock-based compensation - Stock-based Compensation in Opex (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2019	Sep. 28, 2018	Sep. 27, 2019	Sep. 28, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	$ 4,096	$ 5,433	$ 8,719	$ 14,202
Cost of Revenue [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	410	614	830	1,577
Research and Development Expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	1,120	1,676	2,318	4,298
Selling, General and Administrative Expenses [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	2,566	3,143	5,571	8,327
Operating Expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation	$ 3,686	$ 4,819	$ 7,889	$ 12,625