Business Segments (Balance Sheet Information) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Reinsurance receivables	$ 5,914	$ 6,092
Insurance receivables	2,004	1,726
Deferred acquisition costs	603	552
Insurance reserves
Claim and claim adjustment expenses	24,331	24,303
Unearned premiums	3,681	3,250
Future policy benefits	10,974	9,810
Policyholders’ funds	165	191
CNA Specialty [Member]
Reinsurance receivables	750	852
Insurance receivables	676	670
Deferred acquisition costs	306	300
Insurance reserves
Claim and claim adjustment expenses	6,853	6,840
Unearned premiums	1,698	1,629
Future policy benefits
Policyholders’ funds	13	15
CNA Commercial [Member]
Reinsurance receivables	1,131	1,188
Insurance receivables	1,108	1,047
Deferred acquisition costs	278	252
Insurance reserves
Claim and claim adjustment expenses	11,226	11,509
Unearned premiums	1,604	1,480
Future policy benefits
Policyholders’ funds	13	10
Hardy [Member]
Reinsurance receivables	238
Insurance receivables	206
Deferred acquisition costs	19
Insurance reserves
Claim and claim adjustment expenses	466
Unearned premiums	243
Future policy benefits
Policyholders’ funds
Life and Group Non-Core [Member]
Reinsurance receivables	1,297	1,375
Insurance receivables	10	8
Deferred acquisition costs
Insurance reserves
Claim and claim adjustment expenses	2,956	2,825
Unearned premiums	137	141
Future policy benefits	10,974	9,810
Policyholders’ funds	139	166
Corporate and Other Non-Core [Member]
Reinsurance receivables	2,498	2,677
Insurance receivables	4	1
Deferred acquisition costs
Insurance reserves
Claim and claim adjustment expenses	2,830	3,129
Unearned premiums
Future policy benefits
Policyholders’ funds
Eliminations [Member]
Reinsurance receivables
Insurance receivables
Deferred acquisition costs
Insurance reserves
Claim and claim adjustment expenses
Unearned premiums	(1)
Future policy benefits
Policyholders’ funds