Other assets - Components of Other Assets (Details) - USD ($) $ in Millions	Jul. 29, 2017	Jan. 28, 2017	Jul. 30, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Deferred ESP selling costs	$ 87.2	$ 86.1	$ 82.6
Investments (available-for-sale)	28.5	27.2	27.0
Other assets	49.4	51.8	48.4
Total other assets	165.1	165.1	158.0
Deferred costs related to the sale of the extended service plan	$ 29.7	$ 29.4	$ 27.7