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Investments (Details) - Available for Sale Equity Securities - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Investment disclosure details    
Equity securities available for sale, cost $ 440  
Gross gains 20  
Gross losses 7  
Equity securities, available for sale, at fair value 453  
Proceeds from sales of equity securities classified as available for sale 765 $ 92
Equity securities [Member]    
Investment disclosure details    
Gross realized gains 239 17
Gross realized losses 3 $ 3
Public common stock [Member]    
Investment disclosure details    
Equity securities available for sale, cost 332  
Gross gains 8  
Gross losses 1  
Equity securities, available for sale, at fair value 339  
Non-redeemable preferred stock [Member]    
Investment disclosure details    
Equity securities available for sale, cost 108  
Gross gains 12  
Gross losses 6  
Equity securities, available for sale, at fair value $ 114