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Marketable Securities (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Cash equivalents and marketable securities    
Maximum maturity period of available-for-sale securities (in years) 3 years 3 years
Maximum remaining maturity period of available-for-sale securities (in years) 1 year 1 year
Fair Value $ 198,382 $ 183,915
Amortized Cost 198,406 183,926
Gross Unrealized Gains 0 4
Gross Unrealized Losses (24) (15)
Cash, fair value 16,446 19,415
Cash 16,446 19,415
Cash, cash equivalents and marketable securities    
Fair Value 214,828 203,330
Amortized Cost 214,852 203,341
Gross Unrealized Gain 0 4
Gross Unrealized Losses (24) (15)
Money market funds
   
Cash equivalents and marketable securities    
Fair Value 139,768 126,570
Amortized Cost 139,768 126,570
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Weighted Rate of Return (as a percent) 0.01% 0.01%
Corporate notes, bonds and commercial paper
   
Cash equivalents and marketable securities    
Fair Value 58,614 57,345
Amortized Cost 58,638 57,356
Gross Unrealized Gains 0 4
Gross Unrealized Losses (24) (15)
Weighted Rate of Return (as a percent) 0.15% 0.17%
Cash equivalents and marketable securities, Continuous unrealized loss position    
Less than one year, Fair Value 58,590 51,819
Unrealized gain (loss), Gross    
Less than one year, Gross Unrealized Loss (24) (15)
Cash equivalents
   
Cash equivalents and marketable securities    
Fair Value 140,769 129,569
Short term marketable securities
   
Cash equivalents and marketable securities    
Fair Value $ 57,613 $ 54,346