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Available-For-Sale Securities (Summary Of Available-For-Sale Debt Securities By Contractual Maturity) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Investments, Debt and Equity Securities [Abstract]    
Less Than One Year $ 518,248  
Greater Than One Year But Less Than Five Years 0  
Greater than five years but less than ten years, Amortized Cost 0  
Greater than ten years, Amortized Cost 51,500  
Total, Amortized Cost 569,748  
Less than one year, Fair Value 518,248  
Greater than one year but less than five years, Fair Value 0  
Greater than five years but less than ten years, Fair Value 0  
Greater than ten years, Fair Value 48,168  
Total, Fair Value $ 566,416 $ 9,072,646