FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair value disclosure
Cash and cash equivalents	$ 303,954	$ 201,909
Short-term investments	65,408	116,579
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term investments not FDIC-insured	156,400	66,200
Cash deposits
Fair value disclosure
Cash and cash equivalents	240,687	166,619
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	29,066	14,750
Money market funds
Fair value disclosure
Cash and cash equivalents	34,201	20,540
Certificates of deposit
Fair value disclosure
Short-term investments	53,297	69,314
U.S. Treasury securities
Fair value disclosure
Short-term investments	$ 12,111	$ 47,265