Business Segments (Balance Sheet Information) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Reinsurance receivables	$ 6,231	$ 6,092
Insurance receivables	1,983	1,726
Deferred acquisition costs	598	552
Goodwill	154	123
Insurance reserves
Claim and claim adjustment expenses	24,763	24,303
Unearned premiums	3,610	3,250
Future policy benefits	11,475	9,810
Policyholders’ funds	157	191
CNA Specialty [Member]
Reinsurance receivables	665	852
Insurance receivables	673	670
Deferred acquisition costs	300	300
Goodwill	117	123
Insurance reserves
Claim and claim adjustment expenses	6,748	6,840
Unearned premiums	1,685	1,629
Future policy benefits
Policyholders’ funds	8	15
CNA Commercial [Member]
Reinsurance receivables	1,155	1,188
Insurance receivables	1,116	1,047
Deferred acquisition costs	269	252
Goodwill
Insurance reserves
Claim and claim adjustment expenses	11,326	11,509
Unearned premiums	1,569	1,480
Future policy benefits
Policyholders’ funds	15	10
Hardy [Member]
Reinsurance receivables	294
Insurance receivables	181
Deferred acquisition costs	29
Goodwill	37
Insurance reserves
Claim and claim adjustment expenses	521
Unearned premiums	222
Future policy benefits
Policyholders’ funds
Life and Group Non-Core [Member]
Reinsurance receivables	1,273	1,375
Insurance receivables	9	8
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,006	2,825
Unearned premiums	134	141
Future policy benefits	11,475	9,810
Policyholders’ funds	134	166
Corporate and Other Non-Core [Member]
Reinsurance receivables	2,844	2,677
Insurance receivables	4	1
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,162	3,129
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
Future policy benefits
Policyholders’ funds