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FAIR VALUE MEASUREMENTS - Summary of valuation of financial instruments measured at fair value on recurring basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Available-for-sale securities    
Available-for-sale securities: $ 13,031 $ 9,330
Total | Fair value on a recurring basis    
Available-for-sale securities    
Available-for-sale securities: 13,031 9,330
Investments measured at Net Asset Value-Trading securities 6,031 5,547
Total | Fair value on a recurring basis | Municipal Obligations    
Available-for-sale securities    
Available-for-sale securities: 13,031 9,330
Total | Fair value on a recurring basis | Corporate Obligations    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Quoted prices in active markets for identical assets, Level 1    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Quoted prices in active markets for identical assets, Level 1 | Municipal Obligations    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Quoted prices in active markets for identical assets, Level 1 | Corporate Obligations    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Significant other observable inputs, Level 2    
Available-for-sale securities    
Available-for-sale securities: 13,031 9,330
Total | Fair value on a recurring basis | Significant other observable inputs, Level 2 | Municipal Obligations    
Available-for-sale securities    
Available-for-sale securities: 13,031 9,330
Total | Fair value on a recurring basis | Significant other observable inputs, Level 2 | Corporate Obligations    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Significant unobservable inputs, Level 3    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Significant unobservable inputs, Level 3 | Municipal Obligations    
Available-for-sale securities    
Available-for-sale securities: 0 0
Total | Fair value on a recurring basis | Significant unobservable inputs, Level 3 | Corporate Obligations    
Available-for-sale securities    
Available-for-sale securities: $ 0 $ 0