Goodwill and intangibles - Additional Information (Details) - USD ($) $ in Millions		9 Months Ended
Sep. 12, 2017	Oct. 28, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwilll, acquired during period		$ 348.5
R2Net Inc.
Finite-Lived Intangible Assets [Line Items]
Goodwilll, acquired during period	$ 348.5

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Name:	us-gaap_GoodwillAcquiredDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration