FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair value disclosure
Cash and cash equivalents	$ 301,542	$ 303,954
Short-term investments	59,316	65,408
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term investments not FDIC-insured	152,900	156,400
Cash deposits
Fair value disclosure
Cash and cash equivalents	236,380	240,687
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	29,161	29,066
Money market funds
Fair value disclosure
Cash and cash equivalents	36,001	34,201
Certificates of deposit
Fair value disclosure
Short-term investments	$ 59,316	53,297
U.S. Treasury securities
Fair value disclosure
Short-term investments		$ 12,111