Post-retirement benefits - Schedule of changes in the projected benefit obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Pensions
Defined Benefit Plan, Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 827.1	$ 932.9
Employer service cost	5.5	5.3
Plan participants’ contributions	0.2	0.2
Plan amendments	0.0	11.4
Interest cost	23.4	23.6	$ 30.4
Net actuarial loss (gain)	83.1	(41.1)
Benefits paid	(51.6)	(56.5)
Expenses paid from assets	(2.3)	(1.8)
Curtailments and settlements	(3.6)	(13.4)
Foreign currency translation	16.3	(33.5)
Benefit obligation at the end of the period	898.1	827.1	932.9
Accumulated benefit obligation	892.4	821.3
Other post-retirement benefits
Defined Benefit Plan, Change in Benefit Obligation
Benefit obligation at the beginning of the period	60.0	68.4
Interest cost	2.3	2.2	2.7
Net actuarial loss (gain)	(1.8)	(4.8)
Benefits paid	(3.5)	(4.1)
Foreign currency translation	0.9	(1.7)
Benefit obligation at the end of the period	57.9	60.0	$ 68.4
Accumulated benefit obligation	$ 57.9	$ 60.0