Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,720	$ 4,491
Insurance receivables	2,413	2,129
Deferred acquisition costs	620	598
Goodwill	147	150
Insurance reserves
Claim and claim adjustment expenses	22,975	22,663
Unearned premiums	3,865	3,671
Future policy benefits	11,140	10,152
Specialty [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	856	724
Insurance receivables	1,005	890
Deferred acquisition costs	308	307
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,414	6,269
Unearned premiums	1,871	1,839
Future policy benefits	0	0
Commercial [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	621	639
Insurance receivables	1,120	993
Deferred acquisition costs	228	213
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,973	9,183
Unearned premiums	1,417	1,297
Future policy benefits	0	0
International [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	128	144
Insurance receivables	274	233
Deferred acquisition costs	84	78
Goodwill	30	33
Insurance reserves
Claim and claim adjustment expenses	1,409	1,347
Unearned premiums	450	415
Future policy benefits	0	0
Life and Group Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	492	497
Insurance receivables	11	11
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,382	3,220
Unearned premiums	128	120
Future policy benefits	11,140	10,152
Corporate and Other Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	2,623	2,487
Insurance receivables	3	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,797	2,644
Unearned premiums	0	0
Future policy benefits	0	0
Eliminations [Member]
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