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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2014
Fair value disclosure        
Cash and cash equivalents $ 152,236 $ 164,973 $ 192,016 $ 157,042
Short-term investments 64,081 61,597    
Restricted cash 961 1,384    
Concentrations of Credit Risk of Financial Instruments        
Cash and cash equivalents which are not FDIC-insured 71,100 69,900    
Cash deposits        
Fair value disclosure        
Cash and cash equivalents 92,503 110,279    
Restricted cash 961 1,384    
Variable rate demand notes        
Fair value disclosure        
Cash and cash equivalents 19,928 29,790    
Money market funds        
Fair value disclosure        
Cash and cash equivalents 39,805 24,904    
Certificates of deposit        
Fair value disclosure        
Short-term investments $ 64,081 $ 61,597