FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash and Cash Equivalents and Short-term Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair value disclosure
Cash and cash equivalents	$ 183,838	$ 190,186
Short-term investments	124,257	106,806
Concentrations of Credit Risk of Financial Instruments
Cash, cash equivalents and short-term investments not FDIC-insured	54,400	94,700
Cash deposits
Fair value disclosure
Cash and cash equivalents	159,249	124,938
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	14,647	19,786
Money market funds
Fair value disclosure
Cash and cash equivalents	9,942	42,470
Certificates of deposit
Fair value disclosure
Short-term investments	75,175	82,949
U.S. Treasury securities
Fair value disclosure
Cash and cash equivalents		2,992
Short-term investments	$ 49,082	$ 23,857