LEASES - Impact of Adoption (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 03, 2020	Jun. 28, 2019	Jul. 03, 2020	Jun. 28, 2019	Dec. 27, 2019
Operating leases:
Right-of-use assets	$ 22,679		$ 22,679		$ 22,297
Lease liability	$ 5,994		$ 5,994		5,550
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Lease liability		Lease liability
Lease liability, less current portion	$ 17,935		$ 17,935		18,411
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Lease liability, less current portion		Lease liability, less current portion
Total lease obligations	$ 23,929		$ 23,929		23,961
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Total lease obligations		Total lease obligations
Finance leases (included in equipment and leasehold improvements, net):
Equipment and leasehold improvements, gross	$ 28,542		$ 28,542		27,334
Accumulated depreciation and amortization	(15,751)		(15,751)		(15,283)
Total equipment and leasehold improvements, net	12,791		12,791		12,051
Finance lease obligations	332		332		375
Finance lease obligations, less current portion	256		256		191
Total lease obligations	$ 588		$ 588		$ 566
Operating Leases	4 years 8 months 23 days		4 years 8 months 23 days		4 years 7 months 2 days
Finance Leases	1 year 11 months 1 day		1 year 11 months 1 day		1 year 5 months 19 days
Operating Leases, discount rate	4.50%		4.50%		5.14%
Finance Leases, discount rate	4.21%		4.21%		4.80%
Rent expense and related charges for common area maintenance for all facility operating leases	$ 1,800	$ 1,200	$ 3,500	$ 2,300
Facilities and facility related	2,642	$ 2,047	5,336	$ 3,819
Equipment under finance lease
Finance leases (included in equipment and leasehold improvements, net):
Equipment and leasehold improvements, gross	2,247		2,247		$ 1,957
Accumulated depreciation and amortization	(1,572)		(1,572)		(1,291)
Total equipment and leasehold improvements, net	$ 675		$ 675		$ 666