Post-retirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
—Employer service cost	$ 1.4	$ 1.4
—Interest cost	6.5	6.6
—Expected return on plan assets	(7.0)	(5.8)
—Amortization of net actuarial gain	(0.1)	(0.2)
—Settlements	(0.7)	0.4
Net periodic benefit cost	0.1	2.4
Contributions	2.5	3.5
Pensions
Defined Benefit Plan Disclosure [Line Items]
—Employer service cost	1.4	1.4
—Interest cost	5.9	6.0
—Expected return on plan assets	(7.0)	(5.8)
—Amortization of net actuarial gain	0.2	0.0
—Settlements	(0.7)	0.4
Net periodic benefit cost	(0.2)	2.0
Contributions	1.5	2.2
Other post-retirement benefits
Defined Benefit Plan Disclosure [Line Items]
—Employer service cost	0.0	0.0
—Interest cost	0.6	0.6
—Expected return on plan assets	0.0	0.0
—Amortization of net actuarial gain	(0.3)	(0.2)
—Settlements	0.0	0.0
Net periodic benefit cost	0.3	0.4
Contributions	$ 1.0	$ 1.3