Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,564	$ 4,290
Insurance receivables	2,642	2,336
Deferred acquisition costs	673	634
Goodwill	147	148
Insurance reserves
Claim and claim adjustment expenses	21,990	22,004
Unearned premiums	4,410	4,029
Future policy benefits	10,667	11,179
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	926	671
Insurance receivables	996	969
Deferred acquisition costs	317	318
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	5,816	5,669
Unearned premiums	2,103	2,020
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	695	654
Insurance receivables	1,311	1,103
Deferred acquisition costs	248	223
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,599	8,764
Unearned premiums	1,608	1,409
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	265	212
Insurance receivables	327	254
Deferred acquisition costs	108	93
Goodwill	30	31
Insurance reserves
Claim and claim adjustment expenses	1,706	1,636
Unearned premiums	564	472
Future policy benefits	0	0
Operating Segments | Life & Group revenues
Segment Reporting Information [Line Items]
Reinsurance receivables	433	438
Insurance receivables	8	8
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,522	3,499
Unearned premiums	136	128
Future policy benefits	10,667	11,179
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,245	2,315
Insurance receivables	0	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,347	2,436
Unearned premiums	0	0
Future policy benefits	0	0
Eliminations
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