Income taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017	Oct. 01, 2016
Deferred tax assets
Accrued expenses and reserves	$ 12,582	$ 5,639
Deferred revenue	11,185	10,317
U.S. net operating loss carryforwards	15,112	18,385
Foreign net operating loss carryforwards	3,414	5,625
Tax credit carryforwards	43,411	22,969
Stock-based compensation	10,368	7,237
Amortization	4,131	3,237
Depreciation	672	0
Other	453	427
Total deferred tax assets	101,328	73,836
Valuation allowance	(95,088)	(72,380)	$ (94,956)	$ (95,882)
Deferred tax assets, net of valuation allowance	6,240	1,456
Deferred tax liabilities
Tax accounting method change	(5,086)	0
Depreciation	0	(515)
Total deferred tax liabilities	(5,086)	(515)
Net deferred tax assets	$ 1,154	$ 941