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FAIR VALUE MEASUREMENT - Assets Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019	Mar. 29, 2020	Dec. 29, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment loss			$ (140,489)
Impairment of finite-lived intangible assets			(34,700)
Goodwill and intangible asset impairment charge	$ 0	$ 0	(175,189)	$ 0
Fair value, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment loss			(140,489)
Impairment of finite-lived intangible assets			(34,700)
Goodwill and intangible asset impairment charge			$ (175,189)
Fair value, nonrecurring \| Quoted prices in active markets for identical assets (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill					$ 0
Client relationships					0
Total					0
Fair value, nonrecurring \| Significant other observable inputs (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill					0
Client relationships					0
Total					0
Fair value, nonrecurring \| Significant unobservable inputs (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill					31,705
Client relationships					14,700
Total					46,405
Fair value, nonrecurring \| Total fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill					31,705
Client relationships					14,700
Total					$ 46,405	$ 221,600