Leases - Components of lease expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Operating lease costs
Fixed	$ 31,718	$ 30,976	$ 63,932	$ 63,119
Short-term	6,377	5,129	12,823	9,477
Variable	9,415	11,441	19,810	18,919
Sub-leases	(248)	(192)	(486)	(409)
Operating lease costs	47,262	47,354	96,079	91,106
Finance lease costs
Amortization of leased assets	1,476	392	2,730	824
Interest on lease liabilities	154	58	303	89
Finance lease costs	1,630	450	3,033	913
Net lease costs	48,892	47,804	99,112	92,019
Cost of sales
Operating lease costs
Fixed	5,888	8,169	13,289	15,857
Short-term	2,497	1,552	5,138	2,991
Variable	2,121	2,093	4,390	4,371
Sub-leases	(121)	(41)	(218)	(125)
Operating lease costs	10,385	11,773	22,599	23,094
Selling, general and administrative expenses
Operating lease costs
Fixed	25,830	22,807	50,643	47,262
Short-term	3,880	3,577	7,685	6,486
Variable	7,294	9,348	15,420	14,548
Sub-leases	(127)	(151)	(268)	(284)
Operating lease costs	36,877	35,581	73,480	68,012
Depreciation and Amortization
Finance lease costs
Amortization of leased assets	1,476	392	2,730	824
Interest on lease liabilities	0	0	0	0
Finance lease costs	1,476	392	2,730	824
Interest
Finance lease costs
Amortization of leased assets	0	0	0	0
Interest on lease liabilities	154	58	303	89
Finance lease costs	$ 154	$ 58	$ 303	$ 89