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Investments (Details 2) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
investment
Dec. 31, 2010
Investments    
Amortized cost, long-term equity securities $ 625 $ 625
Unrealized Gains 719 1,884
Unrealized Losses (684) (130)
Other-Than-Temporary Impairment Loss (880) (880)
Estimated Fair Value, Long-term Equity Securities 625 625
Amortized Cost, Investments 280,939 404,063
Estimated Fair Value, Investments 280,094 404,937
Temporarily impaired investments    
Temporarily impaired securities, Number of Investments 74  
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value 162,065  
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses (684)  
Short-term investments
   
Investments    
Amortized Cost, Debt Securities 278,776 401,900
Unrealized Gains 95 531
Unrealized Losses (684) (130)
Estimated Fair Value 278,187 402,301
Short-term investments | Debt maturities of one year or less
   
Investments    
Amortized Cost, Debt Securities 182,059 346,919
Unrealized Gains 55 360
Unrealized Losses (263) (75)
Estimated Fair Value 181,851 347,204
Short-term investments | Debt maturities of more than one year
   
Investments    
Amortized Cost, Debt Securities 96,717 54,981
Unrealized Gains 40 171
Unrealized Losses (421) (55)
Estimated Fair Value 96,336 55,097
Corporate debt securities
   
Temporarily impaired investments    
Temporarily impaired securities, Number of Investments 59  
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value 118,695  
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses (586)  
Corporate debt securities | Debt maturities of one year or less
   
Investments    
Amortized Cost, Debt Securities 109,842 196,010
Unrealized Gains 13 294
Unrealized Losses (255) (41)
Estimated Fair Value 109,600 196,263
Corporate debt securities | Debt maturities of more than one year
   
Investments    
Amortized Cost, Debt Securities 57,632 47,842
Unrealized Gains 21 167
Unrealized Losses (331) (44)
Estimated Fair Value 57,322 47,965
Debt securities issued by U.S. government agencies
   
Temporarily impaired investments    
Temporarily impaired securities, Number of Investments 10  
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value 33,685  
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses (72)  
Debt securities issued by U.S. government agencies | Debt maturities of one year or less
   
Investments    
Amortized Cost, Debt Securities 53,723 119,890
Unrealized Gains 35 53
Unrealized Losses (5) (34)
Estimated Fair Value 53,753 119,909
Debt securities issued by U.S. government agencies | Debt maturities of more than one year
   
Investments    
Amortized Cost, Debt Securities 26,754 7,139
Unrealized Gains   4
Unrealized Losses (67) (11)
Estimated Fair Value 26,687 7,132
Debt securities issued by the U.S. Treasury | Debt maturities of one year or less
   
Investments    
Amortized Cost, Debt Securities 2,353 24,030
Unrealized Gains 3 10
Estimated Fair Value 2,356 24,040
Debt securities issued by states of the United States and political subdivisions of the states
   
Temporarily impaired investments    
Temporarily impaired securities, Number of Investments 5  
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value 9,685  
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses (26)  
Debt securities issued by states of the United States and political subdivisions of the states | Debt maturities of one year or less
   
Investments    
Amortized Cost, Debt Securities 16,141 6,989
Unrealized Gains 4 3
Unrealized Losses (3)  
Estimated Fair Value 16,142 6,992
Debt securities issued by states of the United States and political subdivisions of the states | Debt maturities of more than one year
   
Investments    
Amortized Cost, Debt Securities 12,331  
Unrealized Gains 19  
Unrealized Losses (23)  
Estimated Fair Value 12,327  
Equity securities
   
Investments    
Unrealized Gains 624 1,353
Other-Than-Temporary Impairment Loss (880) (880)
Amortized Cost, Investments 2,163 2,163
Estimated Fair Value, Investments 1,907 2,636
Current portion of equity securities
   
Investments    
Amortized Cost, Equity Securities 1,538 1,538
Unrealized Gains 624 1,353
Other-Than-Temporary Impairment Loss (880) (880)
Estimated Fair Value $ 1,282 $ 2,011