Pension plans - Components of Net Periodic Pension Cost (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Oct. 28, 2017	Oct. 29, 2016	Oct. 28, 2017	Oct. 29, 2016
Retirement Benefits [Abstract]
Service cost		$ (0.6)	$ (0.5)	$ (1.6)	$ (1.5)
Interest cost		(1.6)	(1.7)	(4.7)	(5.5)
Expected return on UK Plan assets		2.4	2.5	7.1	8.0
Amortization of unrecognized actuarial losses		(0.7)	(0.4)	(2.2)	(1.2)
Amortization of unrecognized net prior service credits		0.4	0.5	1.3	1.5
Net periodic pension (expense) benefit		(0.1)	0.4	(0.1)	1.3
Net curtailment gain and settlement loss	[1]	3.7	0.0	3.7	0.0
Total recognized in net periodic pension benefit and OCI		$ 3.6	$ 0.4	$ 3.6	$ 1.3

[1] These items are included in the computation of net periodic pension benefit.