Summary of significant accounting policies - Recent Accounting Pronouncements Not Yet Adopted (Details) $ in Thousands	Feb. 03, 2019 USD ($)
Expected \| Effect of adoption of ASU 2016-02, Leases (Topic 842)
Effect of adoption of ASU 2016-02, Leases (Topic 842)
Operating lease liability	$ 1,900,000

Present value of lessee's discounted obligation for lease payments from operating lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

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