Stock-based compensation - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018
Number of Options
Beginning balance (in shares)	48,504,182
Granted (in shares)	1,714,328
Exercised (in shares)	(7,925,897)
Forfeited (in shares)	(5,137,045)
Ending balance (in shares)	37,155,568	48,504,182
Options exercisable (in shares)	26,436,074
Options vested and expected to vest (in shares)	35,606,324
Weighted Average Exercise Price
Beginning balance (in dollars per share)	$ 10.33
Granted (in dollars per share)	13.34
Exercised (in dollars per share)	3.98
Forfeited (in dollars per share)	13.48
Ending balance (in dollars per share)	11.39	$ 10.33
Option exercisable (in dollars per share)	10.23
Options vested and expected to vest (in dollars per share)	$ 11.25
Additional Information
Weighted Average Remaining Contractual Term	6 years 4 months	6 years 7 months
Option exercisable - Weighted Average Remaining Contractual Term	5 years 6 months
Options vested and expected to vest - Weighted Average Remaining Contractual Term	6 years 2 months
Aggregate Intrinsic Value	$ 94,288	$ 276,959
Options exercisable - Weighted Average Intrinsic Value	92,281
Options vested and expected to vest - Weighted Average Intrinsic Value	$ 93,836