Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Earned, Net [Abstract]
Direct	$ 32,402	$ 30,755	$ 29,812
Assumed	329	384	400
Ceded	(424)	(513)	(570)
Premiums Earned Net	32,307	30,626	29,642
Reinsurance Recoveries [Abstract]
Reinsurance recoveries	325	540	737
Individual Life Insurance And Annuity Business Sold [Member]
Reinsurance Recoveries [Abstract]
Reinsurance recoveries	259	279	301
Other Subsegments [Member]
Reinsurance Recoveries [Abstract]
Reinsurance recoveries	66	261	436
Short Duration Contracts [Member]
Premiums Earned, Net [Abstract]
Direct	28,654	27,496	26,751
Assumed	199	247	289
Ceded	(150)	(229)	(254)
Premiums Earned Net	28,703	27,514	26,786
Premiums Written, Net [Abstract]
Direct premiums, written versus earned
Assumed premiums, written versus earned
Ceded premiums, written versus earned
Net premiums, written versus earned
Long Duration Contracts [Member]
Premiums Earned, Net [Abstract]
Direct	3,748	3,259	3,061
Assumed	130	137	111
Premiums Earned Net	3,604	3,112	2,856
Long Duration Contracts [Member] | Individual Life Insurance And Annuity Business Sold [Member]
Premiums Earned, Net [Abstract]
Ceded	(143)	(153)	(158)
Long Duration Contracts [Member] | Other Subsegments [Member]
Premiums Earned, Net [Abstract]
Ceded	$ (131)	$ (131)	$ (158)