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REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 27, 2020
Dec. 28, 2019
Revenue from Contract with Customer [Abstract]    
Payment terms 30 days  
Unpaid advanced client billings $ 13,000 $ 27,000
Contract liabilities (customer contract deposits) $ 41,165 $ 33,080
Percentage of unbilled revenue billed during period 75.00%  
Percentage of contract liabilities recognized as revenue during period 75.00%