Investments (Credit Loss Rollforward) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning of period	$ 170	$ 198	$ 192	$ 211
Addition:
Additional impairments — credit loss OTTI on securities previously impaired	0	9	0	14
Reduction:
Sales (maturities, pay downs or prepayments) of securities previously impaired as credit loss OTTI	(5)	(10)	(27)	(28)
Balance, end of period	$ 165	$ 197	$ 165	$ 197