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Income taxes - Reconciliation of Statutory Tax Rate to Effective Tax Rate (Details)
6 Months Ended
Jul. 29, 2017
Jul. 30, 2016
Income Tax Disclosure [Abstract]    
Forecasted annual effective tax rate 23.00% 25.80%
Discrete items recognized 0.50% 0.00%
Effective tax rate recognized in income statement 23.50% 25.80%