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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair value disclosure    
Cash and cash equivalents $ 102,569 $ 114,280
Short-term investments 54,148 56,838
Restricted cash   962
Concentrations of Credit Risk of Financial Instruments    
Cash and cash equivalents which are not FDIC-insured 46,000 39,900
Cash deposits    
Fair value disclosure    
Cash and cash equivalents 64,143 92,520
Restricted cash   962
Variable rate demand notes    
Fair value disclosure    
Cash and cash equivalents 13,212 16,057
Money market funds    
Fair value disclosure    
Cash and cash equivalents 25,214 5,703
Certificates of deposit    
Fair value disclosure    
Short-term investments $ 54,148 $ 56,838