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REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 28, 2019
Dec. 29, 2018
Revenue from Contract with Customer [Abstract]    
Payment terms 30 days  
Unpaid advanced client billings $ 21,000  
Contract liabilities (customer contract deposits) $ 36,006 $ 38,245
Percentage of unbilled revenue billed during period 85.00%  
Percentage of contract liabilities recognized as revenue during period 80.00%