Stockholders' equity - Textual information (Details)	Aug. 06, 2018 $ / shares shares	Sep. 28, 2019 $ / shares shares	Sep. 03, 2019 USD ($)	Sep. 29, 2018 $ / shares shares
Equity [Abstract]
Preferred stock automatically converted into common stock (in shares)	32,482,590
Convertible preferred stock, conversion ratio	1
Common stock, authorized (in shares)	500,000,000	500,000,000		151,458,824
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001		$ 0.001
Preferred stock, authorized (in shares)	10,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001
Stock repurchase program, authorized value | $			$ 50,000,000.0