Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,614	$ 4,491
Insurance receivables	2,281	2,129
Deferred acquisition costs	619	598
Goodwill	146	150
Insurance reserves
Claim and claim adjustment expenses	22,672	22,663
Unearned premiums	3,862	3,671
Future policy benefits	11,219	10,152
Specialty [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	805	724
Insurance receivables	991	890
Deferred acquisition costs	313	307
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,278	6,269
Unearned premiums	1,922	1,839
Future policy benefits	0	0
Commercial [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	635	639
Insurance receivables	1,018	993
Deferred acquisition costs	223	213
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,879	9,183
Unearned premiums	1,373	1,297
Future policy benefits	0	0
International [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	130	144
Insurance receivables	249	233
Deferred acquisition costs	83	78
Goodwill	29	33
Insurance reserves
Claim and claim adjustment expenses	1,408	1,347
Unearned premiums	432	415
Future policy benefits	0	0
Life and Group Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	483	497
Insurance receivables	21	11
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,407	3,220
Unearned premiums	135	120
Future policy benefits	11,219	10,152
Corporate and Other Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	2,561	2,487
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,700	2,644
Unearned premiums	0	0
Future policy benefits	0	0
Eliminations [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	0	0
Insurance receivables	0	0
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	0	0
Unearned premiums	0	0
Future policy benefits	$ 0	$ 0