Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,730	$ 4,491
Insurance receivables	2,201	2,129
Deferred acquisition costs	622	598
Goodwill	150	150
Insurance reserves
Claim and claim adjustment expenses	23,018	22,663
Unearned premiums	3,807	3,671
Future policy benefits	10,500	10,152
Specialty [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	765	724
Insurance receivables	884	890
Deferred acquisition costs	309	307
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,325	6,269
Unearned premiums	1,861	1,839
Future policy benefits	0	0
Commercial [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	623	639
Insurance receivables	1,025	993
Deferred acquisition costs	225	213
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	9,095	9,183
Unearned premiums	1,347	1,297
Future policy benefits	0	0
International [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	139	144
Insurance receivables	276	233
Deferred acquisition costs	88	78
Goodwill	33	33
Insurance reserves
Claim and claim adjustment expenses	1,395	1,347
Unearned premiums	464	415
Future policy benefits	0	0
Life and Group Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	496	497
Insurance receivables	14	11
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,311	3,220
Unearned premiums	136	120
Future policy benefits	10,500	10,152
Corporate and Other Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	2,707	2,487
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,892	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	(1)	0
Future policy benefits	$ 0	$ 0