Leases - Components of lease expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Operating lease costs
Fixed	$ 32,214	$ 32,143
Short-term	6,446	4,348
Variable	10,395	7,478
Sub-leases	(238)	(217)
Operating lease costs	48,817	43,752
Finance lease costs
Amortization of leased assets	1,254	432
Interest on lease liabilities	149	31
Finance lease costs	1,403	463
Net lease costs	50,220	44,215
Cost of sales
Operating lease costs
Fixed	7,401	7,688
Short-term	2,641	1,439
Variable	2,269	2,278
Sub-leases	(97)	(84)
Operating lease costs	12,214	11,321
Selling, general and administrative expenses
Operating lease costs
Fixed	24,813	24,455
Short-term	3,805	2,909
Variable	8,126	5,200
Sub-leases	(141)	(133)
Operating lease costs	36,603	32,431
Depreciation and Amortization
Finance lease costs
Amortization of leased assets	1,254	432
Interest on lease liabilities	0	0
Finance lease costs	1,254	432
Net lease costs
Interest
Finance lease costs
Amortization of leased assets	0	0
Interest on lease liabilities	149	31
Finance lease costs	149	31
Net lease costs