Leases - Components of lease expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 28, 2019	Sep. 28, 2019
Operating lease costs
Fixed	$ 33,384	$ 96,503
Short-term	3,732	13,209
Variable	8,528	27,447
Sub-leases	(227)	(636)
Operating lease costs	45,417	136,523
Finance lease costs
Amortization of leased assets	404	1,228
Interest on lease liabilities	102	191
Finance lease costs	506	1,419
Net lease costs	45,923	137,942
Cost of sales
Operating lease costs
Fixed	8,068	23,925
Short-term	1,092	4,083
Variable	1,475	5,846
Sub-leases	(114)	(239)
Operating lease costs	10,521	33,615
Finance lease costs
Amortization of leased assets	0	0
Interest on lease liabilities	0	0
Finance lease costs	0	0
Net lease costs	10,521	33,615
Selling, general and administrative expenses
Operating lease costs
Fixed	25,316	72,578
Short-term	2,640	9,126
Variable	7,053	21,601
Sub-leases	(113)	(397)
Operating lease costs	34,896	102,908
Finance lease costs
Amortization of leased assets	404	1,228
Interest on lease liabilities	102	191
Finance lease costs	506	1,419
Net lease costs	$ 35,402	$ 104,327