SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Repayments of notes	$ (4,300)	$ 0
Convertible notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value of notes, beginning of year			4,300
Fair value of notes, end of year	0	4,300	4,300
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	970	7,600
Short-term investments	243,088	202,024
Long-term investments	3,047	3,117
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	4,300
Total carrying value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash equivalents	970	7,600
Short-term investments	243,088	202,024
Long-term investments	$ 3,047	$ 7,417