Investments (Components of other-than-temporary impairment losses recognized in earnings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OTTI losses recognized in earnings	$ 14	$ 81	$ 156
Corporate and other bonds
OTTI losses recognized in earnings	12	59	104
States, municipalities and political subdivisions
OTTI losses recognized in earnings	0	0	18
Residential mortgage-backed
OTTI losses recognized in earnings	1	10	8
Other asset-backed
OTTI losses recognized in earnings	0	3	1
Total asset-backed
OTTI losses recognized in earnings	1	13	9
Total fixed maturity securities available-for-sale
OTTI losses recognized in earnings	13	72	131
Equity securities available-for-sale
OTTI losses recognized in earnings	$ 1	$ 9	$ 25