Changes in accumulated other comprehensive income (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 27, 2020	Jun. 29, 2019	Dec. 28, 2019
Accumulated Other Comprehensive Income [Roll Forward]
Beginning of Period		$ (350,933)	$ (276,476)
Other comprehensive income before reclassification		(49,888)	(23,723)
Amounts reclassified from accumulated other comprehensive income		5,261	(1,550)
Net current-period other comprehensive income		(44,627)	(25,262)
End of Period		(395,560)	(301,738)
Accumulated Other Comprehensive (income) Loss, Defined Benefit Plan, Tax		(46,000)		$ (36,800)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Tax		(16,200)		$ (5,500)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax		(10,300)	(8,000)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax		2,600	(100)
Accounting Standards Update 2017-12
Accumulated Other Comprehensive Income [Roll Forward]
Adoption of ASU 2017-12 (2)	[1]		11
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Beginning of Period		(105,705)	(112,771)
Other comprehensive income before reclassification		(29,064)	(1,762)
Amounts reclassified from accumulated other comprehensive income		0	0
Net current-period other comprehensive income		(29,064)	(1,762)
End of Period		(134,769)	(114,533)
Accumulated Translation Adjustment [Member] | Accounting Standards Update 2017-12
Accumulated Other Comprehensive Income [Roll Forward]
Adoption of ASU 2017-12 (2)	[1]		0
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income [Roll Forward]
Beginning of Period		(228,336)	(170,938)
Other comprehensive income before reclassification		(12,317)	9
Amounts reclassified from accumulated other comprehensive income		0	0
Net current-period other comprehensive income		12,317	(9)
End of Period		(216,019)	(170,947)
Accumulated Defined Benefit Plans Adjustment | Accounting Standards Update 2017-12
Accumulated Other Comprehensive Income [Roll Forward]
Adoption of ASU 2017-12 (2)	[1]		0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent | Interest Rate Swap
Accumulated Other Comprehensive Income [Roll Forward]
Beginning of Period		(16,892)	7,233
Other comprehensive income before reclassification		(33,141)	(21,952)
Amounts reclassified from accumulated other comprehensive income		5,261	(1,550)
Net current-period other comprehensive income		(27,880)	(23,491)
End of Period		$ (44,772)	(16,258)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent | Interest Rate Swap | Accounting Standards Update 2017-12
Accumulated Other Comprehensive Income [Roll Forward]
Adoption of ASU 2017-12 (2)	[1]		$ 11

[1] As of December 30, 2018, the company adopted ASU No. 2017-12, "Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities" using the modified retrospective method. The adoption of this guidance resulted in the recognition of less than $0.1 million as an adjustment to the opening balance of retained earnings.