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INVESTMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale
Amortized Cost	$ 246,937	$ 545,771
Gross Unrealized Holding Gains	643	285
Gross Unrealized Holding Losses	145	88
Fair Value	247,435	545,968
Continuous Unrealized Loss Position less than 12 Months	145	88
Short-term | Commercial paper
Available-for-sale
Amortized Cost	57,107	83,478
Fair Value	57,107	83,478
Short-term | Certificates of deposit
Available-for-sale
Amortized Cost		28,049
Fair Value		28,049
Short-term | Municipal securities
Available-for-sale
Amortized Cost	95,819	147,983
Gross Unrealized Holding Gains	114	145
Gross Unrealized Holding Losses	113	20
Fair Value	95,820	148,108
Continuous Unrealized Loss Position less than 12 Months	113	20
Short-term | U.S. government agency securities
Available-for-sale
Amortized Cost	21,441	40,620
Gross Unrealized Holding Gains	62	5
Gross Unrealized Holding Losses		35
Fair Value	21,503	40,590
Continuous Unrealized Loss Position less than 12 Months		35
Short-term | U.S. treasuries
Available-for-sale
Amortized Cost	46,055	211,055
Gross Unrealized Holding Gains	399	134
Gross Unrealized Holding Losses		31
Fair Value	46,454	211,158
Continuous Unrealized Loss Position less than 12 Months		31
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost	12,628	21,680
Fair Value	12,628	21,680
Long-term | Municipal securities
Available-for-sale
Amortized Cost	6,292	1,562
Gross Unrealized Holding Gains	4
Gross Unrealized Holding Losses	32	1
Fair Value	6,264	1,561
Continuous Unrealized Loss Position less than 12 Months	32	1
Long-term | U.S. government agency securities
Available-for-sale
Amortized Cost	3,512	5,267
Gross Unrealized Holding Gains	10
Gross Unrealized Holding Losses		1
Fair Value	3,522	5,266
Continuous Unrealized Loss Position less than 12 Months		1
Long-term | U.S. treasuries
Available-for-sale
Amortized Cost	4,083	6,077
Gross Unrealized Holding Gains	54	1
Fair Value	$ 4,137	$ 6,078

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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