Net income (loss) per share attributable to common stockholders - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 28, 2020	Dec. 28, 2019	Mar. 30, 2019	Dec. 29, 2018	Mar. 28, 2020	Mar. 30, 2019
Numerator:
Net income	$ (52,320)	$ 70,775	$ (22,824)	$ 61,667	$ 18,454	$ 38,843
Denominator:
Weighted-average shares of common stock - basic (in shares)	109,515,049		102,331,529		109,249,866	101,239,817
Weighted-average shares of common stock - diluted (in shares)	109,515,049		102,331,529		117,819,569	111,474,057
Net income (loss) attributable to common stockholders
Net income (loss) per share attributable to common stockholders - basic (in USD per share)	$ (0.48)		$ (0.22)		$ 0.17	$ 0.38
Net income (loss) per share attributable to common stockholders - diluted (in USD per share)	$ (0.48)		$ (0.22)		$ 0.16	$ 0.35
Stock Options
Denominator:
Effect of potentially dilutive stock options and RSUs (in shares)	0		0		6,185,155	9,901,628
Restricted Stock Units
Denominator:
Effect of potentially dilutive stock options and RSUs (in shares)	0		0		2,384,548	332,612