Leases - Components of lease expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Operating lease costs
Fixed	$ 31,702	$ 33,384	$ 95,634	$ 96,503
Short-term	7,040	3,732	19,863	13,209
Variable	8,910	8,528	28,720	27,447
Sub-leases	(242)	(227)	(728)	(636)
Operating lease costs	47,410	45,417	143,489	136,523
Finance lease costs
Amortization of leased assets	1,672	404	4,402	1,228
Interest on lease liabilities	189	102	492	191
Finance lease costs	1,861	506	4,894	1,419
Net lease costs	49,271	45,923	148,383	137,942
Cost of sales
Operating lease costs
Fixed	5,797	8,068	19,086	23,925
Short-term	3,036	1,092	8,174	4,083
Variable	1,926	1,475	6,316	5,846
Sub-leases	(95)	(114)	(313)	(239)
Operating lease costs	10,664	10,521	33,263	33,615
Selling, general and administrative expenses
Operating lease costs
Fixed	25,905	25,316	76,548	72,578
Short-term	4,004	2,640	11,689	9,126
Variable	6,984	7,053	22,404	21,601
Sub-leases	(147)	(113)	(415)	(397)
Operating lease costs	36,746	34,896	110,226	102,908
Depreciation and Amortization
Finance lease costs
Amortization of leased assets	1,672	404	4,402	1,228
Interest on lease liabilities	0	0	0	0
Finance lease costs	1,672	404	4,402	1,228
Interest
Finance lease costs
Amortization of leased assets	0	0	0	0
Interest on lease liabilities	189	102	492	191
Finance lease costs	$ 189	$ 102	$ 492	$ 191