Fair value measurements - Outstanding Debt (Details) - USD ($) $ in Millions	Nov. 03, 2018	Feb. 03, 2018	Oct. 28, 2017
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt	$ 696.9	$ 718.0	$ 724.3
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt	681.7	722.5	731.7
Senior Notes | Level 2 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt	395.1	394.5	394.3
Senior Notes | Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt	377.8	396.3	398.8
Term Loan | Level 2 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt	301.8	323.5	330.0
Term Loan | Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt	$ 303.9	$ 326.2	$ 332.9