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CASH AND CASH EQUIVALENTS (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and cash equivalents [abstract]      
Cash $ 131,589 $ 181,421  
Short-term investments 328,275 145,706  
Cash and cash equivalents $ 459,864 $ 327,127 $ 211,862