FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash and Cash Equivalents and Short-term Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair value disclosure
Cash and cash equivalents	$ 181,731	$ 190,186
Short-term investments	117,657	106,806
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term investments not FDIC-insured	68,900	94,700
Cash deposits
Fair value disclosure
Cash and cash equivalents	145,629	124,938
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	14,536	19,786
Money market funds
Fair value disclosure
Cash and cash equivalents	21,566	42,470
Certificates of deposit
Fair value disclosure
Short-term investments	81,685	82,949
U.S. Treasury securities
Fair value disclosure
Cash and cash equivalents		2,992
Short-term investments	$ 35,972	$ 23,857