FAIR VALUE MEASUREMENTS - Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Sep. 26, 2018	Dec. 27, 2017
Gain (Loss) on Investments [Line Items]
Fair Value (less than 12 months)	$ 61,800	$ 62,599
Unrealized Loss (less than 12 months)	(60)	(73)
Fair Value (12 months or greater)	0	162
Unrealized Loss (12 months or greater)	0	(3)
Fair Value	61,800	62,761
Unrealized Loss	(60)	(76)
Mutual funds
Gain (Loss) on Investments [Line Items]
Fair Value (less than 12 months)	61,800	60,924
Unrealized Loss (less than 12 months)	(60)	(61)
Fair Value (12 months or greater)	0	0
Unrealized Loss (12 months or greater)	0	0
Fair Value	61,800	60,924
Unrealized Loss	(60)	(61)
Corporate debt securities
Gain (Loss) on Investments [Line Items]
Fair Value (less than 12 months)	0	1,675
Unrealized Loss (less than 12 months)	0	(12)
Fair Value (12 months or greater)	0	162
Unrealized Loss (12 months or greater)	0	(3)
Fair Value	0	1,837
Unrealized Loss	$ 0	$ (15)