FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash and Cash Equivalents and Short-term Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair value disclosure
Cash and cash equivalents	$ 352,165	$ 201,909
Short-term investments	178,810	116,579
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term investments not FDIC-insured	245,900	66,200
Cash deposits
Fair value disclosure
Cash and cash equivalents	224,193	166,619
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	14,846	14,750
Money market funds
Fair value disclosure
Cash and cash equivalents	113,126	20,540
Certificates of deposit
Fair value disclosure
Short-term investments	131,477	69,314
U.S. Treasury securities
Fair value disclosure
Short-term investments	$ 47,333	$ 47,265