Accumulated other comprehensive income (loss) - Reclassifications out of AOCI (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Aug. 04, 2018	Jul. 29, 2017	Aug. 04, 2018	Jul. 29, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales:		$ 929.9	$ 941.7	$ 1,925.7	$ 1,853.9
Interest expense reclassification adjustment		9.4	13.5	18.3	26.1
Income before income taxes		67.0	(122.1)	649.5	(224.8)
Income taxes		(44.0)	28.7	(129.9)	52.9
Net income		23.0	(93.4)	519.6	(171.9)
Amounts reclassified from AOCI		(0.1)	(0.9)	(0.2)	(2.0)
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes		(0.4)	(1.4)	(0.9)	(3.3)
Income taxes		0.2	0.3	0.4	0.8
Net income		(0.2)	(1.1)	(0.5)	(2.5)
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income | Foreign currency contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales:		0.4	(1.2)	0.7	(2.2)
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income | Interest rate swaps
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest expense reclassification adjustment		(0.4)	0.1	(0.7)	0.4
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income | Commodity contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales:		(0.4)	(0.3)	(0.9)	(1.5)
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification adjustment from AOCI, pension and other postretirement benefit plans, before tax	[1]	0.0	0.8	0.3	1.5
Prior service credits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification adjustment from AOCI, pension and other postretirement benefit plans, before tax	[1]	0.1	(0.5)	0.0	(0.9)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification adjustment from AOCI, pension and other postretirement benefit plans, before tax		0.1	0.3	0.3	0.6
Reclassification adjustment from AOCI, pension and other postretirement benefit plans, tax		0.0	(0.1)	0.0	(0.1)
Amounts reclassified from AOCI		$ 0.1	$ 0.2	$ 0.3	$ 0.5

[1] Amounts reclassified from AOCI 13 weeks ended 26 weeks ended (in millions)August 4, 2018 July 29, 2017 August 4, 2018 July 29, 2017 Income statement caption(Gains) losses on cash flow hedges: Foreign currency contracts$0.4 $(1.2) $0.7 $(2.2) Cost of sales (see Note 16)Interest rate swaps(0.4) 0.1 (0.7) 0.4 Interest expense, net (see Note 16)Commodity contracts(0.4) (0.3) (0.9) (1.5) Cost of sales (see Note 16)Total before income tax(0.4) (1.4) (0.9) (3.3) Income taxes0.2 0.3 0.4 0.8 Net of tax(0.2) (1.1) (0.5) (2.5) Defined benefit pension plan items: Amortization of unrecognized actuarial losses— 0.8 0.3 1.5 Other non-operating incomeAmortization of unrecognized net prior service credits0.1 (0.5) — (0.9) Other non-operating incomeTotal before income tax0.1 0.3 0.3 0.6 Income taxes— (0.1) — (0.1) Net of tax0.1 0.2 0.3 0.5 Total reclassifications, net of tax$(0.1) $(0.9) $(0.2) $(2.0)