FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair value disclosure
Cash and cash equivalents	$ 159,307	$ 120,772
Short-term investments	68,013	56,401
Concentrations of Credit Risk of Financial Instruments
Cash and cash equivalents which are not FDIC-insured	92,600	61,100
Cash deposits
Fair value disclosure
Cash and cash equivalents	106,924	86,510
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	15,041	19,744
Money market funds
Fair value disclosure
Cash and cash equivalents	37,342	14,518
Certificates of deposit
Fair value disclosure
Short-term investments	$ 68,013	$ 56,401