SIGNIFICANT ACCOUNTING POLICIES - Adopted Pronouncements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ (1,996,065)		$ (1,954,132)
Operating lease liabilities	59,157
ROU assets	$ 49,579
ASU 2016-02 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities		$ 61,200
ROU assets		$ 51,600