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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Available-for-sale Securities (Details)
12 Months Ended
Dec. 31, 2015
USD ($)
item
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES      
Restricted cash $ 0 $ 0  
Number of objectives of the entity's investment strategy | item 2    
Minimum contractual term of certain current investments which can be liquidated 1 year    
Realized gains $ 14,205 11,000 $ 9,639
Available-for-sale securities      
Amortized Cost 266,188,000 224,740,000  
Gains in Accumulated Other Comprehensive Income 2,000 45,000  
Losses in Accumulated Other Comprehensive Income (446,000) (160,000)  
Estimated Fair Value 265,744,000 224,625,000  
Corporate bonds      
Available-for-sale securities      
Amortized Cost 179,471,000 141,239,000  
Gains in Accumulated Other Comprehensive Income 2,000 20,000  
Losses in Accumulated Other Comprehensive Income (262,000) (135,000)  
Estimated Fair Value 179,211,000 141,124,000  
U.S. government agency securities      
Available-for-sale securities      
Amortized Cost 7,057,000 18,687,000  
Gains in Accumulated Other Comprehensive Income   8,000  
Losses in Accumulated Other Comprehensive Income (18,000) (7,000)  
Estimated Fair Value 7,039,000 18,688,000  
Asset backed securities      
Available-for-sale securities      
Amortized Cost 77,661,000 60,821,000  
Gains in Accumulated Other Comprehensive Income   17,000  
Losses in Accumulated Other Comprehensive Income (166,000) (18,000)  
Estimated Fair Value 77,495,000 60,820,000  
Certificates of deposit      
Available-for-sale securities      
Amortized Cost 1,999,000    
Estimated Fair Value $ 1,999,000    
Commercial paper      
Available-for-sale securities      
Amortized Cost   3,993,000  
Estimated Fair Value   $ 3,993,000