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FAIR VALUE MEASUREMENTS - SECURITIES WITH UNREALIZED LOSSES (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	$ 77,168	$ 66,534
Less Than 12 Months - Gross Unrealized Losses	(169)	(97)
12 Months or Greater - Estimated Fair Value	13,959	6,944
12 Months or Greater - Gross Unrealized Losses	(42)	(10)
Total - Estimated Fair Value	91,127	73,478
Total - Gross Unrealized Losses	(211)	(107)
Government-sponsored enterprise securities (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value		4,997
Less Than 12 Months - Gross Unrealized Losses		(3)
12 Months or Greater - Estimated Fair Value	12,460
12 Months or Greater - Gross Unrealized Losses	(40)
Total - Estimated Fair Value	12,460	4,997
Total - Gross Unrealized Losses	(40)	(3)
Government-sponsored enterprise securities (due in one to two years)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value		12,458
Less Than 12 Months - Gross Unrealized Losses		(42)
Total - Estimated Fair Value		12,458
Total - Gross Unrealized Losses		(42)
Corporate notes (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	55,210	39,218
Less Than 12 Months - Gross Unrealized Losses	(106)	(37)
12 Months or Greater - Estimated Fair Value	1,499	6,944
12 Months or Greater - Gross Unrealized Losses	(2)	(10)
Total - Estimated Fair Value	56,709	46,162
Total - Gross Unrealized Losses	(108)	(47)
Commercial paper (due in less than one year)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	7,717	8,365
Less Than 12 Months - Gross Unrealized Losses	(1)	(5)
Total - Estimated Fair Value	7,717	8,365
Total - Gross Unrealized Losses	(1)	(5)
Corporate notes (due in one to two years)
Available for sale securities with unrealized losses
Less Than 12 Months - Estimated Fair Value	14,241	1,496
Less Than 12 Months - Gross Unrealized Losses	(62)	(10)
Total - Estimated Fair Value	14,241	1,496
Total - Gross Unrealized Losses	$ (62)	$ (10)

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)
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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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
-Publisher FASB
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+ References

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