Share-Based Compensation - Schedule of stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 28, 2019	Dec. 31, 2018
Number of Stock Options
Outstanding, beginning balance (in shares)	5,120,293
Granted (in shares)	1,493,105
Exercised (in shares)	(442,604)
Cancelled/Forfeited (in shares)	(83,625)
Outstanding, ending balance (in shares)	6,087,169	5,120,293
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 3.13
Granted (in dollars per share)	36.61
Exercised (in dollars per share)	2.06
Cancelled/Forfeited (in dollars per share)	8.67
Outstanding, ending balance (in dollars per share)	$ 11.34	$ 3.13
Aggregate Intrinsic Value
Outstanding, beginning balance	$ 81,371
Granted	$ 0
Exercised	$ 33,440
Cancelled/Forfeited	$ 0
Outstanding, ending balance	$ 838,191	$ 81,371
Weighted Average Remaining Contractual Life (Years)
Weighted average remaining contractual life, outstanding	7 years 3 months 18 days	7 years 3 months 18 days
Weighted average remaining contractual life, vested and exercisable	5 years 8 months 12 days
Weighted average remaining contractual life, vested and expected to vest	6 years 7 months 6 days
Number of Stock Options, Vested and exercisable (in shares)	3,233,150
Number of Stock Options, Vested and expected to vest (in shares)	4,503,505
Weighted Average Exercise Price, Vested and exercisable (in dollars per share)	$ 1.72
Weighted Average Exercise Price, Vested and expected to vest (in dollars per share)	$ 6.27
Aggregate intrinsic value, vested and exercisable	$ 474,955
Aggregate intrinsic value, vested and expected to vest	$ 641,908