Share-Based Compensation - Schedule of stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 30, 2019	Dec. 31, 2018
Number of Stock Options
Outstanding, beginning balance (in shares)	5,120,293
Granted (in shares)	0
Exercised (in shares)	(169,583)
Cancelled/Forfeited (in shares)	(50,382)
Outstanding, ending balance (in shares)	4,900,328	5,120,293
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 3.13
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	2.16
Cancelled/Forfeited (in shares)	4.34
Outstanding, ending balance (in dollars per share)	$ 3.15	$ 3.13
Aggregate Intrinsic Value
Outstanding at December 31, 2018	$ 81,371
Granted	$ 0
Exercised	$ 2,859
Cancelled/Forfeited	$ 0
Outstanding at March 30, 2019	$ 80,076	$ 81,371
Stock Option Activity, Additional Disclosures
Weighted average remaining contractual life, outstanding	7 years	7 years 4 months
Weighted average remaining contractual life, vested and exercisable	5 years 10 months 24 days
Weighted average remaining contractual life, vested and expected to vest	5 years 10 months 24 days
Vested and exercisable (in shares)	2,981,633
Vested and expected to vest (in shares)	3,967,483
Vested and exercisable (in dollars per share)	$ 1.11
Vested and expected to vest (in dollars per share)	$ 2.16
Aggregate intrinsic value, vested and exercisable	$ 54,793
Aggregate intrinsic value, vested and expected to vest	$ 68,759