Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,453	$ 4,491
Insurance receivables	2,255	2,129
Deferred acquisition costs	600	598
Goodwill	145	150
Insurance reserves
Claim and claim adjustment expenses	22,343	22,663
Unearned premiums	3,762	3,671
Future policy benefits	10,326	10,152
Specialty [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	760	724
Insurance receivables	982	890
Deferred acquisition costs	310	307
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,149	6,269
Unearned premiums	1,911	1,839
Future policy benefits	0	0
Commercial [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	621	639
Insurance receivables	1,021	993
Deferred acquisition costs	214	213
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,894	9,183
Unearned premiums	1,323	1,297
Future policy benefits	0	0
International [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	131	144
Insurance receivables	233	233
Deferred acquisition costs	76	78
Goodwill	28	33
Insurance reserves
Claim and claim adjustment expenses	1,328	1,347
Unearned premiums	396	415
Future policy benefits	0	0
Life and Group Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	462	497
Insurance receivables	17	11
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,358	3,220
Unearned premiums	132	120
Future policy benefits	10,326	10,152
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	2,479	2,487
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,614	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	0	0
Future policy benefits	$ 0	$ 0