Business Segments - Balance Sheet Information (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,369	$ 4,453
Insurance receivables	2,340	2,255
Deferred acquisition costs	643	600
Goodwill	147	145
Insurance reserves
Claim and claim adjustment expenses	22,209	22,343
Unearned premiums	4,060	3,762
Future policy benefits	11,040	10,326
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	819	760
Insurance receivables	984	982
Deferred acquisition costs	318	310
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,063	6,149
Unearned premiums	2,010	1,911
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	590	621
Insurance receivables	1,087	1,021
Deferred acquisition costs	230	214
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,630	8,894
Unearned premiums	1,426	1,323
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	227	131
Insurance receivables	256	233
Deferred acquisition costs	95	76
Goodwill	30	28
Insurance reserves
Claim and claim adjustment expenses	1,640	1,328
Unearned premiums	490	396
Future policy benefits	0	0
Operating Segments | Life & Group Non-Core
Segment Reporting Information [Line Items]
Reinsurance receivables	453	462
Insurance receivables	11	17
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,468	3,358
Unearned premiums	134	132
Future policy benefits	11,040	10,326
Operating Segments | Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Reinsurance receivables	2,280	2,479
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,408	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