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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Dec. 31, 2014
Fair value disclosure        
Cash and cash equivalents $ 134,788 $ 164,973 $ 166,022 $ 157,042
Short-term investments 69,608 61,597    
Restricted cash 1,385 1,384    
Concentrations of Credit Risk of Financial Instruments        
Cash and cash equivalents which are not FDIC-insured 55,300 69,900    
Cash deposits        
Fair value disclosure        
Cash and cash equivalents 87,820 110,279    
Restricted cash 1,385 1,384    
Variable rate demand notes        
Fair value disclosure        
Cash and cash equivalents 19,863 29,790    
Money market funds        
Fair value disclosure        
Cash and cash equivalents 27,105 24,904    
Certificates of deposit        
Fair value disclosure        
Short-term investments $ 69,608 $ 61,597