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Investments - Write Downs and Sales (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Pre-Tax Asset Write-downs [Line Items]    
Pre-Tax Asset Write-Downs $ 17 $ 10
Sales Information For Available For Sale Fixed Maturities Equity Securities [Abstract]    
Proceeds from sales 499 414
Gross gains on sales 5 47
Gross losses on sales $ (22) $ (2)