Business Segments (Balance Sheet Information) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 5,803	$ 6,231
Insurance receivables	2,083	1,983
Deferred acquisition costs	642	598
Goodwill	154	154
Insurance reserves
Claim and claim adjustment expenses	23,962	24,763
Unearned premiums	3,820	3,610
Future policy benefits	10,681	11,475
Policyholders’ funds	127	157
CNA Specialty [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	613	665
Insurance receivables	746	673
Deferred acquisition costs	320	300
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,846	6,748
Unearned premiums	1,811	1,685
Future policy benefits
Policyholders’ funds	10	8
CNA Commercial [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	1,107	1,155
Insurance receivables	1,112	1,116
Deferred acquisition costs	270	269
Goodwill
Insurance reserves
Claim and claim adjustment expenses	10,763	11,326
Unearned premiums	1,604	1,569
Future policy benefits
Policyholders’ funds	15	15
Hardy [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	227	294
Insurance receivables	211	181
Deferred acquisition costs	52	29
Goodwill	37	37
Insurance reserves
Claim and claim adjustment expenses	432	521
Unearned premiums	274	222
Future policy benefits
Policyholders’ funds
Life and Group Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	1,229	1,273
Insurance receivables	10	9
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,050	3,006
Unearned premiums	132	134
Future policy benefits	10,681	11,475
Policyholders’ funds	102	134
Corporate and Other Non-Core [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables	2,627	2,844
Insurance receivables	4	4
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	2,871	3,162
Unearned premiums
Future policy benefits
Policyholders’ funds
Eliminations [Member]
Segment Reporting Information [Line Items]
Reinsurance receivables
Insurance receivables
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses
Unearned premiums	(1)
Future policy benefits
Policyholders’ funds