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Investments (Fair Values And Gross Unrealized Losses By Credit Rating Category Of Corporate Obligations) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Investment [Line Items]    
Total, Fair Value $ 2,092 $ 2,387
Total, Unrealized Losses (278) (478)
Corporate Obligations [Member]
    
Investment [Line Items]    
Total, Fair Value 495  
Total, Unrealized Losses (37)  
Corporate Obligations [Member] | Aaa [Member]
    
Investment [Line Items]    
Total, Fair Value 68  
Corporate Obligations [Member] | Aa [Member]
    
Investment [Line Items]    
Total, Fair Value 57  
Total, Unrealized Losses (1)  
Corporate Obligations [Member] | A [Member]
    
Investment [Line Items]    
Total, Fair Value 267  
Total, Unrealized Losses (12)  
Corporate Obligations [Member] | Baa [Member]
    
Investment [Line Items]    
Total, Fair Value 80  
Total, Unrealized Losses (22)  
Corporate Obligations [Member] | Below Investment Grade [Member]
    
Investment [Line Items]    
Total, Fair Value 21  
Corporate Obligations [Member] | Not Rated [Member]
    
Investment [Line Items]    
Total, Fair Value 2  
Total, Unrealized Losses $ (2)