Restructuring and other strategic initiatives - Income Statement Location (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 27, 2020 USD ($) facility	Jun. 29, 2019 USD ($)	Jun. 27, 2020 USD ($) facility	Jun. 29, 2019 USD ($)	Dec. 28, 2019 USD ($)	Dec. 29, 2018 USD ($)
Restructuring Cost and Reserve
Restructuring expenses	$ 22.3	$ 0.6	$ 24.2	$ 3.9
—Impairment of fixed assets	3.7	0.0	3.7	0.0
Total expenses related to other strategic initiatives	0.8	1.9	1.4	1.7
Restructuring reserve	$ 17.2	1.2	17.2	1.2	$ 2.9	$ 2.6
Korean Facility
Restructuring Cost and Reserve
—Impairment of fixed assets			$ 3.6
North America
Restructuring Cost and Reserve
Facilities closed (facility) | facility	2		2
Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	$ 17.2	0.3	$ 19.1	3.6
—Severance expenses included in cost of sales
Restructuring Cost and Reserve
Severance-related expenses (included in cost of sales)	0.5	0.5	0.6	0.5
—Severance expenses included in SG&A
Restructuring Cost and Reserve
Severance-related expenses (included in cost of sales)	0.3	1.4	0.8	1.2
—Severance expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	13.6	0.3	13.8	2.9
—Severance expenses | —Severance expenses included in cost of sales | Korean Facility
Restructuring Cost and Reserve
Restructuring reserve	12.8		12.8
—Consulting expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	0.7	0.7	0.9	1.3
—Other restructuring expenses | Restructuring expenses:
Restructuring Cost and Reserve
Restructuring expenses	2.9	(0.7)	4.4	(0.6)
—Impairment of inventory
Restructuring Cost and Reserve
Restructuring expenses	$ 1.4	$ 0.3	1.4	$ 0.3
—Impairment of inventory | Korean Facility
Restructuring Cost and Reserve
Restructuring expenses			$ 1.4