Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,290	$ 4,453
Insurance receivables	2,336	2,255
Deferred acquisition costs	634	600
Goodwill	148	145
Insurance reserves
Claim and claim adjustment expenses	22,004	22,343
Unearned premiums	4,029	3,762
Future policy benefits	11,179	10,326
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	678	760
Insurance receivables	984	982
Deferred acquisition costs	319	310
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	5,848	6,149
Unearned premiums	2,035	1,911
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	647	621
Insurance receivables	1,088	1,021
Deferred acquisition costs	222	214
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,585	8,894
Unearned premiums	1,394	1,323
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	212	131
Insurance receivables	254	233
Deferred acquisition costs	93	76
Goodwill	31	28
Insurance reserves
Claim and claim adjustment expenses	1,636	1,328
Unearned premiums	472	396
Future policy benefits	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	438	462
Insurance receivables	8	17
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,499	3,358
Unearned premiums	128	132
Future policy benefits	11,179	10,326
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,315	2,479
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,436	2,614
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