FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | $ 200,268 | |
Total cash equivalents and marketable securities | 333,445 | $ 290,752 |
Money Market Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 125,184 | $ 86,660 |
Commercial paper [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 7,993 | |
Marketable securities | 33,947 | $ 36,981 |
Corporate bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | $ 13,001 | 18,024 |
Agency bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | 4,999 |
Marketable securities | $ 141,322 | 132,102 |
Agency discount notes [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | 11,998 | 11,986 |
Recurring [Member] | Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total cash equivalents and marketable securities | 125,184 | 86,660 |
Recurring [Member] | Level 1 [Member] | Money Market Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | $ 125,184 | $ 86,660 |
Recurring [Member] | Level 1 [Member] | Commercial paper [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | |
Marketable securities | | |
Recurring [Member] | Level 1 [Member] | Corporate bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | | |
Recurring [Member] | Level 1 [Member] | Agency bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | |
Marketable securities | | |
Recurring [Member] | Level 1 [Member] | Agency discount notes [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | | |
Recurring [Member] | Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total cash equivalents and marketable securities | $ 208,261 | $ 204,092 |
Recurring [Member] | Level 2 [Member] | Money Market Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | |
Recurring [Member] | Level 2 [Member] | Commercial paper [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | $ 7,993 | |
Marketable securities | 33,947 | $ 36,981 |
Recurring [Member] | Level 2 [Member] | Corporate bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | $ 13,001 | 18,024 |
Recurring [Member] | Level 2 [Member] | Agency bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | 4,999 |
Marketable securities | $ 141,322 | 132,102 |
Recurring [Member] | Level 2 [Member] | Agency discount notes [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | $ 11,998 | $ 11,986 |
Recurring [Member] | Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total cash equivalents and marketable securities | | |
Recurring [Member] | Level 3 [Member] | Money Market Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | |
Recurring [Member] | Level 3 [Member] | Commercial paper [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | |
Marketable securities | | |
Recurring [Member] | Level 3 [Member] | Corporate bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | | |
Recurring [Member] | Level 3 [Member] | Agency bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | | |
Marketable securities | | |
Recurring [Member] | Level 3 [Member] | Agency discount notes [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities | | |