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Marketable Securities (Details) (USD $)
3 Months Ended
Mar. 31, 2012
Y
Dec. 31, 2011
Cash equivalents and marketable securities    
Maximum maturity period of available-for-sale securities (in years) 3  
Fair Value $ 216,558,000 $ 264,667,000
Amortized Cost 216,564,000 264,767,000
Gross Unrealized Gains 1,000  
Gross Unrealized Losses (7,000) (100,000)
Cash, fair value 15,960,000 24,789,000
Cash 15,960,000 24,789,000
Cash, cash equivalents and marketable securities    
Fair Value 232,518,000 289,456,000
Amortized Cost 232,524,000 289,556,000
Gross Unrealized Gains 1,000  
Gross Unrealized Losses (7,000) (100,000)
Fair value of marketable debt securities which have insignificant unrealized losses and will mature within one year 39,600,000  
Contractual maturity:    
Less than one year 216,558,000 264,667,000
Unrealized gain (loss), net    
Less than one year (6,000) (100,000)
Money market funds
   
Cash equivalents and marketable securities    
Fair Value 172,588,000 127,559,000
Amortized Cost 172,588,000 127,559,000
Weighted Rate of Return (as a percent) 0.01% 0.01%
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 43,970,000 137,108,000
Amortized Cost 43,976,000 137,208,000
Gross Unrealized Gains 1,000  
Gross Unrealized Losses (7,000) (100,000)
Weighted Rate of Return (as a percent) 0.13% 0.29%
Cash equivalents
   
Cash equivalents and marketable securities    
Fair Value 172,588,000 137,455,000
Short term marketable securities
   
Cash equivalents and marketable securities    
Maximum remaining maturity period of available-for-sale securities (in years) 1  
Fair Value $ 43,970,000 $ 127,212,000