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FAIR VALUE MEASUREMENTS - Available-for-sale Investments Included in Cash Equivalents (Details) - Cash equivalents - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Adjusted Cost | $ 85,363 | $ 73,633
Gross Unrealized Gains/Losses | (33) | (25)
Fair Value | 85,330 | 73,608
Municipal bonds and notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Adjusted Cost | 12,149 | 13,320
Gross Unrealized Gains/Losses | (7) | (1)
Fair Value | 12,142 | 13,319
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Adjusted Cost | 51,862 | 49,432
Gross Unrealized Gains/Losses | (26) | (24)
Fair Value | 51,836 | 49,408
Financial institution instruments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Adjusted Cost | 21,352 | 10,881
Gross Unrealized Gains/Losses | 0 | 0
Fair Value | $ 21,352 | $ 10,881

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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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