Investments (Credit Loss Rollforward) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning of period	$ 215	$ 277
Additions:
Additional impairments — credit loss OTTI on securities previously impaired	0	2
Reductions:
Sales (maturities, pay downs or prepayments) of securities previously impaired as credit loss OTTI	(36)	(9)
Balance, end of period	$ 179	$ 270