Net income (loss) per share attributable to common stockholders - Antidilutive Securities (Details) - shares	3 Months Ended		6 Months Ended
	Mar. 28, 2020	Mar. 30, 2019	Mar. 28, 2020	Mar. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	41,921,718	44,817,779	34,060,389	36,336,510
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	34,840,568	41,602,128	29,425,147	34,926,238
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	7,081,150	3,215,651	4,635,242	1,410,272