Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 28, 2016	Oct. 30, 2015	Oct. 28, 2016	Oct. 30, 2015
Income taxes
Reserves for uncertain tax benefits	$ 6.3		$ 6.3
Interest accrued related to uncertain tax benefits	1.0		1.0
Potential penalties accrued related to uncertain tax benefits	0.9		0.9
Reserves for uncertain tax benefits included in noncurrent Other liabilities	8.2		8.2
Reserve for uncertain tax positions for which a reduction is reasonably possible in the next twelve months	5.2		5.2
Reserve for uncertain tax positions that would impact effective tax rate if recognized	$ 6.3		$ 6.3
Effective income tax rates (as a percent)	36.20%	37.00%	36.10%	37.60%