Income taxes - Valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Valuation Allowance [Roll Forward]
Beginning balance	$ 72,380	$ 94,956	$ 95,882
Increase (decrease) during the period	22,708	(22,576)	(926)
Ending balance	$ 95,088	$ 72,380	$ 94,956

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant