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FAIR VALUE MEASUREMENTS (Details) - Cash and Cash Equivalents - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Adjusted Cost | $ 77,515 | $ 73,633
Gross Unrealized Losses | (18) | (25)
Fair Value | 77,497 | 73,608
Municipal bonds and notes |  |
Debt Securities, Available-for-sale [Line Items] |  |
Adjusted Cost | 553 | 13,320
Gross Unrealized Losses | 0 | (1)
Fair Value | 553 | 13,319
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Adjusted Cost | 35,857 | 49,432
Gross Unrealized Losses | (18) | (24)
Fair Value | 35,839 | 49,408
Financial institution instruments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Adjusted Cost | 41,105 | 10,881
Gross Unrealized Losses | 0 | 0
Fair Value | $ 41,105 | $ 10,881

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