Investments - Write Downs and Sales (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pre-Tax Asset Write-downs [Line Items]
Pre-Tax Asset Write-Downs			$ 19	$ 44
Sales Information For Available For Sale Fixed Maturities Equity Securities [Abstract]
Proceeds from sales	$ 446	$ 300	1,376	1,012
Gross gains on sales	26	9	85	43
Gross losses on sales	$ 3	$ 0	$ 7	$ 6