Income taxes - Effective tax reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
U.S. federal income taxes at statutory rate	$ (226)	$ (3,570)	$ (5,778)
U.S. state and local income taxes	(9,315)	(1,441)	(2,454)
Foreign income tax rate differential	129	(53)	(1,101)
Dutch tax settlement	0	0	7,361
Stock-based compensation	(2,399)	4,025	1,503
Federal research tax credits	(8,418)	(4,333)	(2,978)
Unrecognized federal tax benefits	(2,806)	1,990	1,191
Change in tax rate	1,161	25,725	0
Net Impact of GILTI	239	0	0
BEAT	781	0	0
Other	822	259	1,197
Change in valuation allowance	23,722	(21,546)	(1,232)
Provision for (benefit from) income taxes	$ 3,690	$ 1,056	$ (2,291)