Revenue recognition - Rollforward (Details) - Sterling Jewelers - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Nov. 03, 2018	Oct. 28, 2017	Nov. 03, 2018	Oct. 28, 2017
Movement in Deferred Revenue [Roll Forward]
ESP deferred revenue, beginning of period	$ 906.6	$ 902.2	$ 916.1	$ 905.6
Plans sold	72.3	72.9	259.2	266.3
Revenue recognized	(86.1)	(92.3)	(282.5)	(289.1)
ESP deferred revenue, end of period	$ 892.8	$ 882.8	$ 892.8	$ 882.8