Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,437	$ 4,290
Insurance receivables	2,414	2,336
Deferred acquisition costs	665	634
Goodwill	149	148
Insurance reserves
Claim and claim adjustment expenses	22,067	22,004
Unearned premiums	4,256	4,029
Future policy benefits	10,783	11,179
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	737	671
Insurance receivables	947	969
Deferred acquisition costs	318	318
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	5,696	5,669
Unearned premiums	2,063	2,020
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	690	654
Insurance receivables	1,140	1,103
Deferred acquisition costs	241	223
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,718	8,764
Unearned premiums	1,490	1,409
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	244	212
Insurance receivables	315	254
Deferred acquisition costs	106	93
Goodwill	32	31
Insurance reserves
Claim and claim adjustment expenses	1,718	1,636
Unearned premiums	559	472
Future policy benefits	0	0
Operating Segments | Life & Group revenues
Segment Reporting Information [Line Items]
Reinsurance receivables	436	438
Insurance receivables	11	8
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,501	3,499
Unearned premiums	145	128
Future policy benefits	10,783	11,179
Operating Segments | Corporate & Other revenues
Segment Reporting Information [Line Items]
Reinsurance receivables	2,330	2,315
Insurance receivables	1	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,434	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