Income taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
May 02, 2020	Oct. 31, 2020
Income Tax Disclosure [Abstract]
CARES Act, income tax benefit on Tax Cuts and Jobs Act		$ 73.8
CARES Act, income tax benefit on net operating losses		$ 23.5
Increase in valuation allowance	$ 66.9

Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration