Pension plans - Components of Net Periodic Pension Cost (Details) - Foreign Pension Plan, Defined Benefit - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 29, 2016	Oct. 31, 2015	Oct. 29, 2016	Oct. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ (0.5)	$ (0.7)	$ (1.5)	$ (2.0)
Interest cost	(1.7)	(1.9)	(5.5)	(5.8)
Expected return on UK Plan assets	2.5	2.9	8.0	8.7
Amortization of unrecognized actuarial losses	(0.4)	(0.9)	(1.2)	(2.6)
Amortization of unrecognized net prior service credits	0.5	0.6	1.5	1.7
Net periodic pension benefit	$ 0.4	$ 0.0	$ 1.3	$ 0.0