Business Segments (Balance Sheet Information) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Reinsurance receivables	$ 6,137	$ 6,231
Insurance receivables	2,072	1,983
Deferred acquisition costs	641	598
Goodwill	151	154
Insurance reserves
Claim and claim adjustment expenses	24,511	24,763
Unearned premiums	3,759	3,610
Future policy benefits	11,469	11,475
Policyholders’ funds	154	157
CNA Specialty [Member]
Reinsurance receivables	662	665
Insurance receivables	707	673
Deferred acquisition costs	313	300
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	6,807	6,748
Unearned premiums	1,770	1,685
Future policy benefits
Policyholders’ funds	9	8
CNA Commercial [Member]
Reinsurance receivables	1,170	1,155
Insurance receivables	1,168	1,116
Deferred acquisition costs	287	269
Goodwill
Insurance reserves
Claim and claim adjustment expenses	11,105	11,326
Unearned premiums	1,629	1,569
Future policy benefits
Policyholders’ funds	14	15
Hardy [Member]
Reinsurance receivables	251	294
Insurance receivables	181	181
Deferred acquisition costs	41	29
Goodwill	34	37
Insurance reserves
Claim and claim adjustment expenses	470	521
Unearned premiums	218	222
Future policy benefits
Policyholders’ funds
Life and Group Non-Core [Member]
Reinsurance receivables	1,273	1,273
Insurance receivables	13	9
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,031	3,006
Unearned premiums	143	134
Future policy benefits	11,469	11,475
Policyholders’ funds	131	134
Corporate and Other Non-Core [Member]
Reinsurance receivables	2,781	2,844
Insurance receivables	3	4
Deferred acquisition costs
Goodwill
Insurance reserves
Claim and claim adjustment expenses	3,098	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