Derivative financial instruments - Gain Loss From Derivative Instruments Not Designated As Hedging Instruments (Details) - Not designated as hedging instrument - USD ($) $ in Millions	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative, recognized in the income statement	$ 0.8	$ 2.9
Currency forward contracts | SG&A
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative, recognized in the income statement	0.3	2.7
Currency swap | Other expense (income)
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative, recognized in the income statement	$ 0.5	$ 0.2