Post-retirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
—Employer service cost	$ 1.4	$ 1.4
—Interest cost	5.0	6.5
—Expected return on plan assets	(5.5)	(7.0)
—Net amortization of prior period losses (gains)	0.0	(0.1)
—Settlements and curtailments	0.0	(0.7)
Net periodic benefit cost	0.9	0.1
Contributions	2.9	2.5
Pensions
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
—Employer service cost	1.4	1.4
—Interest cost	4.6	5.9
—Expected return on plan assets	(5.5)	(7.0)
—Net amortization of prior period losses (gains)	0.3	0.2
—Settlements and curtailments	0.0	(0.7)
Net periodic benefit cost	0.8	(0.2)
Contributions	1.7	1.5
Other post-retirement benefits
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
—Employer service cost	0.0	0.0
—Interest cost	0.4	0.6
—Expected return on plan assets	0.0	0.0
—Net amortization of prior period losses (gains)	(0.3)	(0.3)
—Settlements and curtailments	0.0	0.0
Net periodic benefit cost	0.1	0.3
Contributions	$ 1.2	$ 1.0