Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,253	$ 4,290
Insurance receivables	2,400	2,336
Deferred acquisition costs	654	634
Goodwill	147	148
Insurance reserves
Claim and claim adjustment expenses	21,604	22,004
Unearned premiums	4,289	4,029
Future policy benefits	10,605	11,179
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	685	671
Insurance receivables	942	969
Deferred acquisition costs	315	318
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	5,509	5,669
Unearned premiums	2,129	2,020
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	729	654
Insurance receivables	1,189	1,103
Deferred acquisition costs	240	223
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,584	8,764
Unearned premiums	1,520	1,409
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	239	212
Insurance receivables	259	254
Deferred acquisition costs	99	93
Goodwill	30	31
Insurance reserves
Claim and claim adjustment expenses	1,689	1,636
Unearned premiums	510	472
Future policy benefits	0	0
Operating Segments | Life & Group revenues
Segment Reporting Information [Line Items]
Reinsurance receivables	427	438
Insurance receivables	10	8
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,551	3,499
Unearned premiums	130	128
Future policy benefits	10,605	11,179
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,173	2,315
Insurance receivables	0	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,271	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	0	0
Future policy benefits	$ 0	$ 0