Post-retirement benefits - Narratives (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 30, 2023	Dec. 31, 2022	Jan. 01, 2022
Defined Benefit Plan Disclosure
Benefit obligation, period increase	$ 9.9
Initial health care cost trend (percentage)	5.76%	5.97%
Ultimate health care cost trend (percentage)	4.84%	4.85%
Pensions
Defined Benefit Plan Disclosure
Defined contribution expense	$ 21.0	$ 20.2	$ 20.6
Interest cost	25.1	15.2	13.4
Actuarial gain (loss)	(6.7)	180.6
Actual loss on plan assets	19.3	(217.5)
Expected future employer contributions in current fiscal year	7.3
Other post-retirement benefits
Defined Benefit Plan Disclosure
Interest cost	1.5	1.2	$ 1.2
Actuarial gain (loss)	(0.3)	$ 13.9
Expected future employer contributions in current fiscal year	$ 3.0