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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair value disclosure
Cash and cash equivalents	$ 362,619	$ 303,954
Short-term investments	59,967	65,408
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term investments not FDIC-insured	132,500	156,400
Cash deposits
Fair value disclosure
Cash and cash equivalents	284,265	240,687
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	29,253	29,066
Money market funds
Fair value disclosure
Cash and cash equivalents	49,101	34,201
Certificates of deposit
Fair value disclosure
Short-term investments	$ 59,967	53,297
U.S. Treasury securities
Fair value disclosure
Short-term investments		$ 12,111

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The amount of cash and cash equivalents as of the balance sheet date that is not insured by the Federal Deposit Insurance Corporation.

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