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Investments (Details) - Investment Information, Available for Sale Equity Securities - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2017
Dec. 31, 2017
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 $ 340  
Equity securities [Member]    
Investment disclosure details    
Gross realized gains 146  
Gross realized losses $ 1  
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