CASH EQUIVALENTS AND INVESTMENTS - Short-term investments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Short-term investments	$ 472,203	$ 562,108
U.S. treasury securities
Short-term investments	380,219	470,109
U.S. government agency securities
Short-term investments	62,034	$ 91,999
Commercial Paper
Short-term investments	$ 29,950

Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(4))
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-Subparagraph (SX 210.7-03(a)(1)(g))
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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