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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair value disclosure    
Cash and cash equivalents $ 109,034 $ 114,280
Short-term investments 57,052 56,838
Restricted cash   962
Concentrations of Credit Risk of Financial Instruments    
Cash and cash equivalents which are not FDIC-insured 46,900 39,900
Cash deposits    
Fair value disclosure    
Cash and cash equivalents 67,140 92,520
Restricted cash   962
Variable rate demand notes    
Fair value disclosure    
Cash and cash equivalents 19,677 16,057
Money market funds    
Fair value disclosure    
Cash and cash equivalents 22,217 5,703
Certificates of deposit    
Fair value disclosure    
Short-term investments $ 57,052 $ 56,838