Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Aug. 03, 2018	Aug. 04, 2017	Aug. 03, 2018	Aug. 04, 2017
Income taxes
Reserves for uncertain tax benefits	$ 1.1		$ 1.1
Interest accrued related to uncertain tax benefits	0.8		0.8
Potential penalties accrued related to uncertain tax benefits	0.9		0.9
Reserves for uncertain tax benefits included in noncurrent Other liabilities	2.8		2.8
Reserve for uncertain tax positions that would impact effective tax rate if recognized	$ 1.1		$ 1.1
Effective income tax rates (as a percent)	21.50%	37.20%	21.60%	37.20%
U.S. federal statutory rate on earnings before income taxes (as a percent)	21.00%	35.00%	21.00%	35.00%