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Marketable Securities (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Y
Dec. 31, 2010
Cash equivalents and marketable securities    
Maximum maturity period of available-for-sale securities (in years) 3  
Fair Value $ 264,667,000 $ 494,905,000
Amortized Cost 264,667,000 494,977,000
Gross Unrealized Gains   37,000
Gross Unrealized Losses (100,000) (109,000)
Cash, fair value 24,789,000 17,104,000
Cash 24,789,000 17,104,000
Cash, cash equivalents and marketable securities    
Fair Value 289,456,000 512,009,000
Amortized Cost 289,556,000 512,081,000
Gross Unrealized Gains   37,000
Gross Unrealized Losses (100,000) (109,000)
Fair value of marketable debt securities which have insignificant unrealized losses and will mature within one year 137,100,000  
Unrealized gain (loss), net    
Less than one year (100,000) (72,000)
Contractual maturity:    
Less than one year 264,667,000 494,905,000
Money market funds
   
Cash equivalents and marketable securities    
Fair Value 127,559,000 132,364,000
Amortized Cost 127,559,000 132,364,000
Weighted Rate of Return (as a percent) 0.01% 0.04%
U.S. government bonds and notes
   
Cash equivalents and marketable securities    
Fair Value   266,817,000
Amortized Cost   266,840,000
Gross Unrealized Gains   29,000
Gross Unrealized Losses   (52,000)
Weighted Rate of Return (as a percent)   0.26%
Corporate notes, bonds and commercial paper
   
Cash equivalents and marketable securities    
Maximum maturity period of available-for-sale securities (in years) 3  
Maximum remaining maturity period of available-for-sale securities (in years) 1  
Fair Value 137,108,000 95,724,000
Amortized Cost 137,208,000 95,773,000
Gross Unrealized Gains   8,000
Gross Unrealized Losses (100,000) (57,000)
Weighted Rate of Return (as a percent) 0.29% 0.39%
Cash equivalents
   
Cash equivalents and marketable securities    
Maximum remaining maturity period of available-for-sale securities (in years) 1  
Fair Value 137,455,000 198,158,000
Short term marketable securities
   
Cash equivalents and marketable securities    
Maximum remaining maturity period of available-for-sale securities (in years) 1  
Fair Value $ 127,212,000 $ 296,747,000