Post-retirement benefits - Schedule of changes in the projected benefit obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Defined Benefit Plan, Change in Benefit Obligation
Interest cost	$ 18.3
Net actuarial loss	(31.3)
Pensions
Defined Benefit Plan, Change in Benefit Obligation
Benefit obligation at the beginning of the period	898.1	$ 827.1
Employer service cost	5.6	5.5
Plan participants’ contributions	0.2	0.2
Plan amendments	1.5	0.0
Interest cost	18.3	23.4	$ 23.6
Net actuarial loss	31.3	83.1
Benefits paid	(44.6)	(51.6)
Expenses paid from assets	(2.0)	(2.3)
Curtailments and settlements	(30.6)	(3.6)
Foreign currency translation	28.1	16.3
Benefit obligation at the end of the period	905.9	898.1	827.1
Accumulated benefit obligation	901.6	892.4
Other post-retirement benefits
Defined Benefit Plan, Change in Benefit Obligation
Benefit obligation at the beginning of the period	57.9	60.0
Interest cost	1.7	2.3	2.2
Net actuarial loss	0.3	(1.8)
Benefits paid	(3.7)	(3.5)
Foreign currency translation	0.5	0.9
Benefit obligation at the end of the period	56.7	57.9	$ 60.0
Accumulated benefit obligation	$ 56.7	$ 57.9