Net income (loss) per share attributable to common stockholders - Antidilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Jun. 29, 2019	Jun. 30, 2018	Jun. 29, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	47,310,971	80,906,730	36,941,809	80,906,730
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	0	32,482,590	0	32,482,590
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	40,916,245	48,370,248	34,408,923	48,370,248
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	6,394,726	0	2,532,886	0
Shares subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	0	53,892	0	53,892