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SIGNIFICANT ACCOUNTING POLICIES - Adopted Pronouncements (Details) - USD ($)
$ in Thousands
Sep. 29, 2019
Dec. 31, 2018
Dec. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Accumulated deficit $ (2,318,299)   $ (1,954,132)
Operating lease liabilities 56,493    
ROU assets $ 46,977    
ASU 2016-02 | Restatement Adjustment      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating lease liabilities   $ 61,200  
ROU assets   $ 51,600