SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Repayments of notes	$ 0	$ (4,300)
Convertible notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value of notes, beginning of year	0	4,300
Fair value of notes, end of year	0	0
Fair value measured on a recurring basis | Quoted prices in active markets (Level 1) [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash equivalents	34,980	970
Short-term investments	297,981	243,088
Long-term investments	2,187	3,047
Fair value measured on a recurring basis | Significant other observable inputs (Level 2) [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Long-term investments	0	0
Fair value measured on a recurring basis | Significant unobersvable inputs (Level 3) [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Long-term investments	0	0
Total carrying value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash equivalents	34,980	970
Short-term investments	297,981	243,088
Long-term investments	$ 2,187	$ 3,047