Business Segments (Balance Sheet Information) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Reinsurance receivables	$ 6,041	$ 6,231
Insurance receivables	2,227	1,983
Deferred acquisition costs	650	598
Goodwill	152	154
Insurance reserves
Claim and claim adjustment expenses	24,339	24,763
Unearned premiums	3,869	3,610
Future policy benefits	10,787	11,475
Policyholders’ funds	133	157
CNA Specialty [Member]
Reinsurance receivables	698	665
Insurance receivables	786	673
Deferred acquisition costs	317	300
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,877	6,748
Unearned premiums	1,787	1,685
Future policy benefits
Policyholders’ funds	9	8
CNA Commercial [Member]
Reinsurance receivables	1,146	1,155
Insurance receivables	1,219	1,116
Deferred acquisition costs	277	269
Goodwill
Insurance reserves
Claim and claim adjustment expenses	10,973	11,326
Unearned premiums	1,682	1,569
Future policy benefits
Policyholders’ funds	14	15
Hardy [Member]
Reinsurance receivables	235	294
Insurance receivables	208	181
Deferred acquisition costs	56	29
Goodwill	35	37
Insurance reserves
Claim and claim adjustment expenses	447	521
Unearned premiums	265	222
Future policy benefits
Policyholders’ funds
Life and Group Non-Core [Member]
Reinsurance receivables	1,258	1,273
Insurance receivables	11	9
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,031	3,006
Unearned premiums	136	134
Future policy benefits	10,787	11,475
Policyholders’ funds	110	134
Corporate and Other Non-Core [Member]
Reinsurance receivables	2,704	2,844
Insurance receivables	3	4
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,011	3,162
Unearned premiums
Future policy benefits
Policyholders’ funds
Eliminations [Member]
Reinsurance receivables
Insurance receivables
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses
Unearned premiums	(1)
Future policy benefits
Policyholders’ funds