FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair value disclosure
Cash and cash equivalents	$ 124,652	$ 120,772
Short-term investments	54,669	56,401
Concentrations of Credit Risk of Financial Instruments
Cash and cash equivalents which are not FDIC-insured	69,100	61,100
Cash deposits
Fair value disclosure
Cash and cash equivalents	93,022	86,510
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	17,810	19,744
Money market funds
Fair value disclosure
Cash and cash equivalents	13,820	14,518
Certificates of deposit
Fair value disclosure
Short-term investments	$ 54,669	$ 56,401