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Fair Value of Financial Instruments (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Mar. 31, 2013
Dec. 31, 2012
Fair Value of Financial Instruments        
Cash and cash equivalents, cost $ 2,113 $ 4,364 $ 5,279 $ 2,529
Cash and cash equivalents, fair value 2,113 4,364    
Estimated fair values of financial instruments        
Unrealized Gains 25 23    
Unrealized Losses (21) (10)    
Short-term investments
        
Estimated fair values of financial instruments        
Cost 32,720 32,183    
Unrealized Gains 21 14    
Unrealized Losses (2) (5)    
Fair Value 32,739 32,192    
Short-term investments | U.S. government debt securities
        
Estimated fair values of financial instruments        
Cost 15,943 19,944    
Unrealized Gains 7 11    
Unrealized Losses (1) (1)    
Fair Value 15,949 19,954    
Short-term investments | Corporate notes
        
Estimated fair values of financial instruments        
Cost 12,558 7,245    
Unrealized Gains 13 2    
Unrealized Losses   (2)    
Fair Value 12,571 7,245    
Short-term investments | Certificates of deposit
        
Estimated fair values of financial instruments        
Cost 4,219 4,994    
Unrealized Gains 1 1    
Unrealized Losses (1) (2)    
Fair Value 4,219 4,993    
Long-term investments
        
Estimated fair values of financial instruments        
Cost 10,839 13,922    
Unrealized Gains 4 9    
Unrealized Losses (19) (5)    
Fair Value 10,824 13,926    
Long-term investments | U.S. government debt securities
        
Estimated fair values of financial instruments        
Cost 2,801 3,016    
Unrealized Losses (6)      
Fair Value 2,795 3,016    
Long-term investments | Corporate notes
        
Estimated fair values of financial instruments        
Cost 6,838 9,466    
Unrealized Gains 4 9    
Unrealized Losses (11) (1)    
Fair Value 6,831 9,474    
Long-term investments | Certificates of deposit
        
Estimated fair values of financial instruments        
Cost 1,200 1,440    
Unrealized Losses (2) (4)    
Fair Value $ 1,198 $ 1,436