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REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 27, 2021
Mar. 28, 2020
Dec. 26, 2020
Revenue from Contract with Customer [Abstract]      
Payment terms 30 days    
Unpaid advanced client billings $ 19,000   $ 16,000
Contract liabilities (customer contract deposits) $ 36,652   $ 42,244
Percentage of unbilled revenue billed during period 60.00% 60.00%  
Amount of unbilled revenue billed during period   $ 122,000  
Percentage of contract liabilities recognized as revenue during period 60.00% 60.00%  
Amount of contract liabilities recognized as revenue   $ 193,000