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Goodwill and intangibles - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Sep. 12, 2017
Aug. 04, 2018
Jul. 29, 2017
Aug. 04, 2018
Jul. 29, 2017
Finite-Lived Intangible Assets [Line Items]          
Goodwilll, acquired during period         $ 301.7
Goodwill and intangible impairments   $ 0.0 $ 0.0 $ 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