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Investments (Narrative) (Details) (USD $)
9 Months Ended
Sep. 30, 2013
Securities
Sep. 30, 2012
Dec. 31, 2012
Securities
Investments [Abstract]      
Proceeds from calls of United States Government sponsored agency securities and public and private bonds $ 1,100,000,000 $ 3,800,000,000  
Proceeds from maturities, prepayments and calls of held for investment securities 0 2,618,207,000  
Fixed income securities with call features, percentage 31.00%    
Fixed income securities with call features, callable during next 12 months, percentage 5.00%    
Fixed income securities with call features, callable during next 12 months, amount 1,300,000,000    
United States Government agency securities, callable during next 12 months 500,000,000    
United States Government agency securities, callable during next 12 months, book yield 3.75%    
Percentage of fixed maturity portfolio rated investment grade based on NAIC designations 98.00%   98.00%
Number of individual securities with unrealized losses 885   198
Percentage of unrealized losses on fixed maturity securities where securities are rated investment grade 94.00%   75.00%
Investment Holdings [Line Items]      
Available for sale securities, amortized cost 25,783,116,000   21,957,027,000
Proceeds from sales of available for sale securities 1,200,000,000 276,100,000  
Scheduled principal repayments, calls and tenders for available for sale securities 2,955,734,000 1,942,533,000  
Fixed Maturity Securities [Member]
     
Investment Holdings [Line Items]      
Scheduled principal repayments, calls and tenders for available for sale securities 1,900,000,000 1,600,000,000  
Residential Mortgage Backed Securities [Member] | Residential Mortgage Backed Securities, Alt-A Sector [Member]
     
Investment Holdings [Line Items]      
Available for sale securities, number of securities in unrealized loss positions 36    
Available for sale securities, amortized cost 323,400,000    
Available for sale securities, fair value $ 340,700,000    
Other Asset Backed Securities [Member] | Investments, External Credit Rating, Non Investment Grade [Member]
     
Investment Holdings [Line Items]      
Available for sale securities, number of securities in unrealized loss positions 0