Business Segments (Balance Sheet Information) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Reinsurance receivables	$ 5,989	$ 6,092
Insurance receivables	1,777	1,726
Deferred acquisition costs	576	552
Insurance reserves
Claim and claim adjustment expenses	24,203	24,303
Unearned premiums	3,383	3,250
Future policy benefits	9,959	9,810
Policyholders’ funds	169	191
CNA Specialty [Member]
Reinsurance receivables	863	852
Insurance receivables	679	670
Deferred acquisition costs	311	300
Insurance reserves
Claim and claim adjustment expenses	6,920	6,840
Unearned premiums	1,705	1,629
Future policy benefits
Policyholders’ funds	12	15
CNA Commercial [Member]
Reinsurance receivables	1,159	1,188
Insurance receivables	1,086	1,047
Deferred acquisition costs	265	252
Insurance reserves
Claim and claim adjustment expenses	11,422	11,509
Unearned premiums	1,529	1,480
Future policy benefits
Policyholders’ funds	13	10
Life and Group Non-Core [Member]
Reinsurance receivables	1,359	1,375
Insurance receivables	8	8
Deferred acquisition costs
Insurance reserves
Claim and claim adjustment expenses	2,875	2,825
Unearned premiums	150	141
Future policy benefits	9,959	9,810
Policyholders’ funds	144	166
Corporate and Other Non-Core [Member]
Reinsurance receivables	2,608	2,677
Insurance receivables	4	1
Deferred acquisition costs
Insurance reserves
Claim and claim adjustment expenses	2,986	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