Summary of significant accounting policies - Fiscal Quarter (Details)	9 Months Ended
Nov. 03, 2018	Oct. 28, 2017
Summary of significant accounting policies
Fiscal period	91 days	91 days

Duration of a fiscal period, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but not limited to, weeks in a year or quarter.

No definition available.

Name:	us-gaap_FiscalPeriodDuration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration