Net loss per share attributable to common stockholders - Antidilutive securities excluded from computation of earnings per share (Details) - shares	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation (in shares)	46,447,646	48,504,182	78,353,734
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation (in shares)	42,300,183	48,504,182	45,817,252
RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation (in shares)	4,147,463	0	0
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation (in shares)	0	0	32,482,590
Shares subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation (in shares)	0	0	53,892