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Organization and principal accounting policies - Reclassification (Details) - USD ($) $ in Millions	Apr. 30, 2016	Jan. 30, 2016	May. 02, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Non-current liabilities	$ 88.1	$ 72.5	$ 57.3
New Accounting Pronouncement, Early Adoption, Effect
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current assets			(5.7)
Non-current assets			(117.7)
Current liabilities			(158.9)
Non-current liabilities			$ 35.5