Business Segments - Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,464	$ 4,453
Insurance receivables	2,452	2,255
Deferred acquisition costs	647	600
Goodwill	147	145
Insurance reserves
Claim and claim adjustment expenses	22,179	22,343
Unearned premiums	4,107	3,762
Future policy benefits	10,824	10,326
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	880	760
Insurance receivables	1,020	982
Deferred acquisition costs	318	310
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,229	6,149
Unearned premiums	1,988	1,911
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	591	621
Insurance receivables	1,129	1,021
Deferred acquisition costs	234	214
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,586	8,894
Unearned premiums	1,481	1,323
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	150	131
Insurance receivables	290	233
Deferred acquisition costs	95	76
Goodwill	30	28
Insurance reserves
Claim and claim adjustment expenses	1,442	1,328
Unearned premiums	500	396
Future policy benefits	0	0
Operating Segments | Life & Group Non-Core
Segment Reporting Information [Line Items]
Reinsurance receivables	461	462
Insurance receivables	11	17
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,417	3,358
Unearned premiums	139	132
Future policy benefits	10,824	10,326
Operating Segments | Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Reinsurance receivables	2,382	2,479
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,505	2,614
Unearned premiums	0	0
Future policy benefits	0	0
Intersegment 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