FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash and Cash Equivalents and Short-term Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair value disclosure
Cash and cash equivalents	$ 138,399	$ 190,186
Short-term investments	116,225	106,806
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term deposits not FDIC-insured	65,000	94,700
Cash deposits
Fair value disclosure
Cash and cash equivalents	104,896	124,938
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	14,973	19,786
Money market funds
Fair value disclosure
Cash and cash equivalents	18,530	42,470
Certificates of deposit
Fair value disclosure
Short-term investments	83,425	82,949
U.S. Treasury securities
Fair value disclosure
Cash and cash equivalents		2,992
Short-term investments	$ 32,800	$ 23,857