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Organization and principal accounting policies - Reclassification (Details) - USD ($) $ in Millions	Oct. 29, 2016	Jan. 30, 2016	Oct. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Non-current liabilities	$ 133.4	$ 72.5	$ 56.7
Accounting Standards Update 2015-17
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current assets			(3.6)
Non-current assets			(130.3)
Current liabilities			(170.5)
Non-current liabilities			$ 36.6