Revenue recognition - Rollforward (Details) - Sterling Jewelers - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Aug. 04, 2018	Jul. 29, 2017	Aug. 04, 2018	Jul. 29, 2017
Movement in Deferred Revenue [Roll Forward]
ESP deferred revenue, beginning of period	$ 913.5	$ 903.7	$ 916.1	$ 905.6
Plans sold	90.9	96.8	186.9	193.4
Revenue recognized	(97.8)	(98.3)	(196.4)	(196.8)
ESP deferred revenue, end of period	$ 906.6	$ 902.2	$ 906.6	$ 902.2