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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details)
 3 Months Ended
Mar. 31, 2016
USD ($)
item
Mar. 31, 2015
USD ($)
Dec. 31, 2015
USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES      
Number of objectives of the entity's investment strategy | item 2    
Realized gains $ 3,300 $ 3,000  
Minimum contractual term of certain current investments which can be liquidated 1 year    
Available-for-sale securities      
Amortized Cost $ 210,412,000   $ 266,188,000
Gains in Accumulated Other Comprehensive Income 115,000   2,000
Losses in Accumulated Other Comprehensive Income (86,000)   (446,000)
Estimated Fair Value 210,441,000   265,744,000
Realized losses 0 $ 0  
Corporate bonds      
Available-for-sale securities      
Amortized Cost 132,854,000   179,471,000
Gains in Accumulated Other Comprehensive Income 93,000   2,000
Losses in Accumulated Other Comprehensive Income (41,000)   (262,000)
Estimated Fair Value 132,906,000   179,211,000
Asset backed securities      
Available-for-sale securities      
Amortized Cost 62,380,000   77,661,000
Gains in Accumulated Other Comprehensive Income 12,000    
Losses in Accumulated Other Comprehensive Income (44,000)   (166,000)
Estimated Fair Value 62,348,000   77,495,000
U.S. government agency securities      
Available-for-sale securities      
Amortized Cost 7,057,000   7,057,000
Gains in Accumulated Other Comprehensive Income 8,000    
Losses in Accumulated Other Comprehensive Income     (18,000)
Estimated Fair Value 7,065,000   7,039,000
Certificates of deposit      
Available-for-sale securities      
Amortized Cost 5,500,000   1,999,000
Gains in Accumulated Other Comprehensive Income 2,000    
Estimated Fair Value 5,502,000   $ 1,999,000
Commercial paper      
Available-for-sale securities      
Amortized Cost 2,621,000    
Losses in Accumulated Other Comprehensive Income (1,000)    
Estimated Fair Value $ 2,620,000