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INCOME TAXES - Summary of the activity of unrecognized tax benefits (Details 4) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized tax benefits - January 1 $ 1,037 $ 792 $ 526
Gross increases - tax positions in prior period 120 245 266
Gross decreases - tax positions in prior period (1,157) 0 0
Unrecognized tax benefits - December 31 $ 0 $ 1,037 $ 792