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Organization and principal accounting policies - Reclassification (Details) - USD ($) $ in Millions	Jul. 30, 2016	Jan. 30, 2016	Aug. 01, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Non-current liabilities	$ 79.8	$ 72.5	$ 62.5
New Accounting Pronouncement, Early Adoption, Effect
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current assets			(4.3)
Non-current assets			(125.8)
Current liabilities			(172.4)
Non-current liabilities			$ 42.3