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FAIR VALUE MEASUREMENTS - SECURITIES WITH UNREALIZED LOSSES (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	$ 107,767	$ 77,168
Less Than 12 Months - Gross Unrealized Losses	(176)	(169)
12 Months or Greater - Estimated Fair Value	14,221	13,959
12 Months or Greater - Gross Unrealized Losses	(36)	(42)
Total - Estimated Fair Value	121,988	91,127
Total - Gross Unrealized Losses	(212)	(211)
Commercial paper (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	22,628	7,717
Less Than 12 Months - Gross Unrealized Losses	(29)	(1)
Total - Estimated Fair Value	22,628	7,717
Total - Gross Unrealized Losses	(29)	(1)
Corporate notes (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	66,557	55,210
Less Than 12 Months - Gross Unrealized Losses	(82)	(106)
12 Months or Greater - Estimated Fair Value	14,221	1,499
12 Months or Greater - Gross Unrealized Losses	(36)	(2)
Total - Estimated Fair Value	80,778	56,709
Total - Gross Unrealized Losses	(118)	(108)
Corporate notes (due in one to two years)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	18,582	14,241
Less Than 12 Months - Gross Unrealized Losses	(65)	(62)
Total - Estimated Fair Value	18,582	14,241
Total - Gross Unrealized Losses	$ (65)	(62)
Government-sponsored enterprise securities (due in less than one year)
Available for sale securities with unrealized losses
12 Months or Greater - Estimated Fair Value		12,460
12 Months or Greater - Gross Unrealized Losses		(40)
Total - Estimated Fair Value		12,460
Total - Gross Unrealized Losses		$ (40)

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months.

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-Name Accounting Standards Codification
-Topic 320
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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