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FAIR VALUE MEASUREMENTS Fair Value Measurement (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments at Fair Value $ 223,635 $ 240,553
Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 94,079 179,951
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 127,251 51,122
Government securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value   9,285
Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 2,305 195
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments at Fair Value 2,305 195
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value   0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 2,305 195
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments at Fair Value 221,330 240,358
Significant Other Observable Inputs (Level 2) | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 94,079 179,951
Significant Other Observable Inputs (Level 2) | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value 127,251 51,122
Significant Other Observable Inputs (Level 2) | Government securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value   9,285
Significant Other Observable Inputs (Level 2) | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at Fair Value $ 0 $ 0