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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
2 3/4 percent Convertible Senior Notes due 2019
   
Assets measured at fair value on a recurring basis    
Interest rate on convertible debt (as a percent) 2.75%  
Significant Other Observable Inputs (Level 2) | 2 3/4 percent Convertible Senior Notes due 2019
   
Assets measured at fair value on a recurring basis    
Fair value of convertible notes $ 230,400  
Recurring basis | Total
   
Assets measured at fair value on a recurring basis    
Cash equivalents 82,863 58,892
Total assets 345,063 338,361
Recurring basis | Total | Corporate debt securities
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 190,648 166,922
Recurring basis | Total | Debt securities issued by U.S. government agencies
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 31,151 80,440
Recurring basis | Total | Debt securities issued by the U.S. Treasury
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 13,486 2,356
Recurring basis | Total | Debt securities issued by states of the United States and political subdivisions of the states
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 23,725 28,469
Recurring basis | Total | Equity securities
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 3,190 1,282
Recurring basis | Quoted Prices in Active Markets (Level 1)
   
Assets measured at fair value on a recurring basis    
Cash equivalents 73,064 55,893
Total assets 89,740 59,531
Recurring basis | Quoted Prices in Active Markets (Level 1) | Debt securities issued by the U.S. Treasury
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 13,486 2,356
Recurring basis | Quoted Prices in Active Markets (Level 1) | Equity securities
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 3,190 1,282
Recurring basis | Significant Other Observable Inputs (Level 2)
   
Assets measured at fair value on a recurring basis    
Cash equivalents 9,799 2,999
Total assets 255,323 278,830
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt securities
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 190,648 166,922
Recurring basis | Significant Other Observable Inputs (Level 2) | Debt securities issued by U.S. government agencies
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities 31,151 80,440
Recurring basis | Significant Other Observable Inputs (Level 2) | Debt securities issued by states of the United States and political subdivisions of the states
   
Assets measured at fair value on a recurring basis    
Available-for-sale securities $ 23,725 $ 28,469