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FAIR VALUE MEASUREMENTS - SECURITIES WITH UNREALIZED LOSSES (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	$ 75,089	$ 77,168
Less Than 12 Months - Gross Unrealized Losses	(290)	(169)
12 Months or Longer - Estimated Fair Value	16,497	13,959
12 Months or Longer - Gross Unrealized Losses	(48)	(42)
Total - Estimated Fair Value	91,586	91,127
Total - Gross Unrealized Losses	(338)	(211)
Government-sponsored enterprise securities (due in less than one year)
Available for sale securities with unrealized losses
12 Months or Longer - Estimated Fair Value	9,970	12,460
12 Months or Longer - Gross Unrealized Losses	(30)	(40)
Total - Estimated Fair Value	9,970	12,460
Total - Gross Unrealized Losses	(30)	(40)
Commercial paper (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	8,958	7,717
Less Than 12 Months - Gross Unrealized Losses	(5)	(1)
Total - Estimated Fair Value	8,958	7,717
Total - Gross Unrealized Losses	(5)	(1)
Corporate notes (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	52,947	55,210
Less Than 12 Months - Gross Unrealized Losses	(210)	(106)
12 Months or Longer - Estimated Fair Value	6,527	1,499
12 Months or Longer - Gross Unrealized Losses	(18)	(2)
Total - Estimated Fair Value	59,474	56,709
Total - Gross Unrealized Losses	(228)	(108)
Corporate notes (due in one to two years)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	13,184	14,241
Less Than 12 Months - Gross Unrealized Losses	(75)	(62)
Total - Estimated Fair Value	13,184	14,241
Total - Gross Unrealized Losses	$ (75)	$ (62)

X

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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