FAIR VALUE OF FINANCIAL INSTRUMENTS - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	$ 76,427	$ 0
Long-term securities	15,478	0
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	230,435	337,234
Fair Value, Measurements, Recurring | Level 1 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	19,568	0
Long-term securities	1,503	0
Fair Value, Measurements, Recurring | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	209,364	337,234
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	72,334	0
Fair Value, Measurements, Recurring | Level 2 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	13,088	0
Long-term securities	6,519	0
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	19,787	0
Long-term securities	4,423	0
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	17,455	0
Fair Value, Measurements, Recurring | Level 2 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	6,529
Long-term securities	3,033	0
Fair Value, Measurements, Recurring | Level 2 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,500	0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term securities	0	0
Long-term securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 0	$ 0