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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair value disclosure    
Cash and cash equivalents $ 82,253 $ 114,280
Short-term investments 56,984 56,838
Restricted cash 962 962
Concentrations of Credit Risk of Financial Instruments    
Cash and cash equivalents which are not FDIC-insured 32,600 39,900
Cash deposits    
Fair value disclosure    
Cash and cash equivalents 61,067 92,520
Restricted cash 962 962
Variable rate demand notes    
Fair value disclosure    
Cash and cash equivalents 578 16,057
Money market funds    
Fair value disclosure    
Cash and cash equivalents 20,608 5,703
Certificates of deposit    
Fair value disclosure    
Short-term investments $ 56,984 $ 56,838