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Cash, Cash Equivalents and Available-for-Sale Securities (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Cash and cash equivalents and available-for-sale securities, Amortized Cost        
Available-for-sale securities including cash and cash equivalents, Amortized Cost $ 214,553 $ 146,811    
Less amounts classified as cash equivalents, Amortized Cost (58,605) (39,554) (37,216) (40,533)
Available-for-sale securities, Amortized Cost 155,948 107,257    
Available-for-sale securities, Gross Unrealized Gains 33 27    
Available-for-sale securities, Gross Unrealized Losses   (2)    
Cash and cash equivalents and available-for-sale securities, Estimated Fair Value        
Available-for-sale securities including cash and cash equivalents, Estimated Fair Value 214,586 146,836    
Less amounts classified as cash equivalents, Estimated Fair Value (58,605) (39,554)    
Available-for-sale securities, Estimated Fair Value 155,981 107,282    
Cash and money market funds
       
Cash and cash equivalents and available-for-sale securities, Amortized Cost        
Available-for-sale securities including cash and cash equivalents, Amortized Cost 58,605 38,547    
Cash and cash equivalents and available-for-sale securities, Estimated Fair Value        
Available-for-sale securities including cash and cash equivalents, Estimated Fair Value 58,605 38,547    
U.S. Treasury securities
       
Cash and cash equivalents and available-for-sale securities, Amortized Cost        
Available-for-sale securities including cash and cash equivalents, Amortized Cost 155,948 108,264    
Available-for-sale securities, Gross Unrealized Gains 33 27    
Available-for-sale securities, Gross Unrealized Losses   (2)    
Cash and cash equivalents and available-for-sale securities, Estimated Fair Value        
Available-for-sale securities including cash and cash equivalents, Estimated Fair Value $ 155,981 $ 108,289