Net income (loss) per share attributable to common stockholders - Antidilutive Securities (Details) - shares	3 Months Ended		6 Months Ended
	Mar. 30, 2019	Mar. 31, 2018	Mar. 30, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	44,817,779	77,708,419	36,336,510	63,029,511
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)	41,602,128	45,171,937	34,926,238	30,546,921
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially antidilutive securities (in shares)	3,215,651	0	1,410,272	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	0