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FAIR VALUE MEASUREMENTS - SECURITIES WITH UNREALIZED LOSSES (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Available for sale securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | $ 44,906 | $ 107,767
Less Than 12 Months - Gross Unrealized Losses | (29) | (176)
12 Months or Longer - Estimated Fair Value | 13,226 | 14,221
12 Months or Longer - Gross Unrealized Losses | (8) | (36)
Total - Estimated Fair Value | 58,132 | 121,988
Total - Gross Unrealized Losses | (37) | (212)
Commercial paper (due in less than one year) |  |
Available for sale securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 15,792 | 22,628
Less Than 12 Months - Gross Unrealized Losses | (19) | (29)
Total - Estimated Fair Value | 15,792 | 22,628
Total - Gross Unrealized Losses | (19) | (29)
Corporate notes (due in less than one year) |  |
Available for sale securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 27,115 | 66,557
Less Than 12 Months - Gross Unrealized Losses | (5) | (82)
12 Months or Longer - Estimated Fair Value | 13,226 | 14,221
12 Months or Longer - Gross Unrealized Losses | (8) | (36)
Total - Estimated Fair Value | 40,341 | 80,778
Total - Gross Unrealized Losses | (13) | (118)
Corporate notes (due in one to two years) |  |
Available for sale securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 1,999 | 18,582
Less Than 12 Months - Gross Unrealized Losses | (5) | (65)
Total - Estimated Fair Value | 1,999 | 18,582
Total - Gross Unrealized Losses | $ (5) | $ (65)

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