FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair value disclosure
Cash and cash equivalents	$ 177,436	$ 120,772
Short-term investments	75,879	56,401
Concentrations of Credit Risk of Financial Instruments
Cash and cash equivalents which are not FDIC-insured	85,200	61,100
Cash deposits
Fair value disclosure
Cash and cash equivalents	126,607	86,510
Variable rate demand notes
Fair value disclosure
Cash and cash equivalents	17,980	19,744
Money market funds
Fair value disclosure
Cash and cash equivalents	32,849	14,518
Certificates of deposit
Fair value disclosure
Short-term investments	$ 75,879	$ 56,401