Revenue recognition - Narrative (Details) - North America	Aug. 04, 2018
Extended Service Plans and Lifetime Warranty Agreements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, expected timing of satisfaction, period	17 years
Jewelry Replacement Plan
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, expected timing of satisfaction, period	3 years

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant