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Goodwill and intangibles - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended
Sep. 12, 2017
May 05, 2018
Apr. 29, 2017
Finite-Lived Intangible Assets [Line Items]      
Goodwilll, acquired during period     $ 301.7
Goodwill and intangible impairments   $ 448.7 $ 0.0
Impairment   (308.8)  
R2Net Inc.      
Finite-Lived Intangible Assets [Line Items]      
Goodwilll, acquired during period $ 299.1    
Sterling Jewelers      
Finite-Lived Intangible Assets [Line Items]      
Impairment   23.2  
Zale Jewelry      
Finite-Lived Intangible Assets [Line Items]      
Impairment   285.6  
Impairment of intangible assets (excluding goodwill)   $ 139.9