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Fair Value of Financial Instruments (Details) (USD $)
 3 Months Ended
Mar. 31, 2013
Item
Dec. 31, 2012
Mar. 31, 2012
Dec. 31, 2011
Fair Value of Financial Instruments        
Cash and cash equivalents, cost $ 5,279,000 $ 2,529,000 $ 9,420,000 $ 40,025,000
Cash and cash equivalents, fair value 5,279,000 2,529,000    
Estimated fair values of financial instruments        
Unrealized Gains 25,000 17,000    
Unrealized Losses (10,000) (6,000)    
Fair value of available-for-sale securities with unrealized losses 10,900,000      
Number of maturity groups 2      
Investments by rolling maturity, Cost        
Due within 1 year 25,158,000 30,790,000    
Due in 1-2 years 6,856,000 7,380,000    
Total 32,014,000 38,170,000    
Investments by rolling maturity, Unrealized Gains        
Due within 1 year 24,000 14,000    
Due in 1-2 years 1,000 3,000    
Total 25,000 17,000    
Investments by rolling maturity, Unrealized Losses        
Due within 1 year (2,000) (6,000)    
Due in 1-2 years (8,000)      
Total (10,000) (6,000)    
Investments by rolling maturity, Fair Value        
Due within 1 year 25,180,000 30,798,000    
Due in 1-2 years 6,849,000 7,383,000    
Total 32,029,000 38,181,000    
Short-term investments
        
Estimated fair values of financial instruments        
Cost 25,158,000 30,790,000    
Unrealized Gains 24,000 14,000    
Unrealized Losses (2,000) (6,000)    
Fair Value 25,180,000 30,798,000    
Investments by rolling maturity, Unrealized Gains        
Total 24,000 14,000    
Investments by rolling maturity, Unrealized Losses        
Total (2,000) (6,000)    
Short-term investments | U.S. government debt securities
        
Estimated fair values of financial instruments        
Cost 10,888,000 15,852,000    
Unrealized Gains 6,000 6,000    
Unrealized Losses   (2,000)    
Fair Value 10,894,000 15,856,000    
Investments by rolling maturity, Unrealized Gains        
Total 6,000 6,000    
Investments by rolling maturity, Unrealized Losses        
Total   (2,000)    
Short-term investments | Corporate notes
        
Estimated fair values of financial instruments        
Cost 11,190,000 14,471,000    
Unrealized Gains 16,000 8,000    
Unrealized Losses (1,000) (4,000)    
Fair Value 11,205,000 14,475,000    
Investments by rolling maturity, Unrealized Gains        
Total 16,000 8,000    
Investments by rolling maturity, Unrealized Losses        
Total (1,000) (4,000)    
Short-term investments | Certificates of deposit
        
Estimated fair values of financial instruments        
Cost 3,080,000 467,000    
Unrealized Gains 2,000      
Unrealized Losses (1,000)      
Fair Value 3,081,000 467,000    
Investments by rolling maturity, Unrealized Gains        
Total 2,000      
Investments by rolling maturity, Unrealized Losses        
Total (1,000)      
Long-term investments
        
Estimated fair values of financial instruments        
Cost 6,856,000 7,380,000    
Unrealized Gains 1,000 3,000    
Unrealized Losses (8,000)      
Fair Value 6,849,000 7,383,000    
Investments by rolling maturity, Unrealized Gains        
Total 1,000 3,000    
Investments by rolling maturity, Unrealized Losses        
Total (8,000)      
Long-term investments | U.S. government debt securities
        
Estimated fair values of financial instruments        
Cost 4,507,000 6,330,000    
Unrealized Gains   2,000    
Unrealized Losses (2,000)      
Fair Value 4,505,000 6,332,000    
Investments by rolling maturity, Unrealized Gains        
Total   2,000    
Investments by rolling maturity, Unrealized Losses        
Total (2,000)      
Long-term investments | Corporate notes
        
Estimated fair values of financial instruments        
Cost 2,109,000 1,050,000    
Unrealized Gains   1,000    
Unrealized Losses (6,000)      
Fair Value 2,103,000 1,051,000    
Investments by rolling maturity, Unrealized Gains        
Total   1,000    
Investments by rolling maturity, Unrealized Losses        
Total (6,000)      
Long-term investments | Certificates of deposit
        
Estimated fair values of financial instruments        
Cost 240,000      
Unrealized Gains 1,000      
Fair Value 241,000      
Investments by rolling maturity, Unrealized Gains        
Total $ 1,000