Organization and principal accounting policies - Narrative (Details) - segment	3 Months Ended				12 Months Ended
	Jan. 28, 2017	Oct. 29, 2016	Jul. 30, 2016	Apr. 30, 2016	Jan. 28, 2017
Property, Plant and Equipment [Line Items]
Number of reportable segments				5
Percent of annual sales				20.00%
Scenario, Forecast
Property, Plant and Equipment [Line Items]
Percent of annual sales	40.00%	20.00%	20.00%
Scenario, Forecast | Minimum
Property, Plant and Equipment [Line Items]
Operating income expected in fourth quarter, Percent					45.00%
Scenario, Forecast | Maximum
Property, Plant and Equipment [Line Items]
Operating income expected in fourth quarter, Percent					55.00%
Scenario, Forecast | Sterling Jewelers | Minimum
Property, Plant and Equipment [Line Items]
Operating income expected in fourth quarter, Percent					40.00%
Scenario, Forecast | Sterling Jewelers | Maximum
Property, Plant and Equipment [Line Items]
Operating income expected in fourth quarter, Percent					45.00%