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Long-term and Short-term Debt (Long-term and Short-term Outstanding) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Capital lease obligations $ 9 $ 7
Long-term debt 17,963 16,135
Short-term debt 100 100
Total $ 18,063 16,235
Senior notes    
Debt Instrument [Line Items]    
Interest Rate Range Minimum 1.76%  
Interest Rate Range Maximum 7.72%  
Weighted Average Interest Rate 5.10%  
Debt Instrument, Maturity Date Range, Start Jun. 01, 2016  
Debt Instrument, Maturity Date Range, End May 13, 2046  
Long-term Debt $ 16,994 15,317
Surplus notes    
Debt Instrument [Line Items]    
Interest Rate Range Minimum 7.63%  
Interest Rate Range Maximum 7.88%  
Weighted Average Interest Rate 7.80%  
Debt Instrument, Maturity Date Range, Start Jan. 15, 2024  
Debt Instrument, Maturity Date Range, End Nov. 01, 2025  
Long-term Debt $ 502 701
Other notes    
Debt Instrument [Line Items]    
Interest Rate Range Minimum 1.36%  
Interest Rate Range Maximum 8.00%  
Weighted Average Interest Rate 4.07%  
Debt Instrument, Maturity Date Range, Start Sep. 12, 2016  
Debt Instrument, Maturity Date Range, End Nov. 30, 2030  
Long-term Debt $ 458 $ 110