Income taxes - Effective tax reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Oct. 01, 2016	Oct. 03, 2015
Income Tax Disclosure [Abstract]
U.S. federal income taxes at statutory rate	$ (3,570)	$ (5,778)	$ (12,450)
U.S. state and local income taxes	(1,441)	(2,454)	(1,813)
Foreign income tax rate differential	(53)	(1,101)	(4,680)
Dutch tax settlement	0	7,361	0
Stock-based compensation	4,025	1,503	6,521
Research tax credits	(2,343)	(1,787)	(4,036)
Change in tax rate	25,725	0	(624)
Other	259	1,197	191
Change in valuation allowance	(21,546)	(1,232)	19,535
Provision for (benefit from) income taxes	$ 1,056	$ (2,291)	$ 2,644	$ 3,242