FAIR VALUE MEASUREMENTS - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	$ 287,446	$ 188,644
Long-term investments	14,794	14,960
Fair Value, Measurements, Recurring [Member] | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,532	71,773
Short-term investments	287,446	188,644
Long-term investments	14,794	14,960
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,532	71,773
Short-term investments	287,446	188,644
Long-term investments	$ 14,794	$ 14,960