Restructuring and other strategic initiatives - Income Statement Location (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 26, 2020 USD ($) facility	Sep. 28, 2019 USD ($)	Sep. 26, 2020 USD ($) facility	Sep. 28, 2019 USD ($)	Dec. 28, 2019 USD ($)	Dec. 29, 2018 USD ($)
Restructuring Cost and Reserve
Restructuring expenses	$ 8.7	$ 2.0	$ 32.9	$ 5.9
—Impairment of fixed assets	1.4	0.7	5.1	0.7
Total expenses related to other strategic initiatives	3.3	4.3	4.7	6.0
Restructuring reserve	$ 11.4	2.4	11.4	2.4	$ 2.9	$ 2.6
Korean Facility
Restructuring Cost and Reserve
—Impairment of fixed assets			4.8
Severance-related expenses (included in cost of sales)			$ 12.8
North America
Restructuring Cost and Reserve
Facilities closed (facility) | facility	2		2
Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	$ 7.3	0.3	$ 26.4	3.9
—Severance expenses included in cost of sales
Restructuring Cost and Reserve
Severance-related expenses (included in cost of sales)	0.3	2.5	0.9	3.0
—Severance expenses included in SG&A
Restructuring Cost and Reserve
Severance-related expenses (included in cost of sales)	3.0	1.8	3.8	3.0
—Severance expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	3.3	0.4	17.1	3.3
—Non-severance labor and benefit expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	1.8	0.0	2.6	0.0
—Consulting expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	0.3	0.1	1.2	1.4
—Other restructuring expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	1.9	(0.2)	5.5	(0.8)
—Impairment of inventory
Restructuring Cost and Reserve
Restructuring expenses	$ 0.0	$ 1.0	1.4	$ 1.3
—Impairment of inventory | Korean Facility
Restructuring Cost and Reserve
Restructuring expenses			$ 1.4