Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating Revenues
Net earned premiums	$ 1,810	$ 1,825	$ 5,427	$ 5,389
Net investment income	480	555	1,556	1,680
Other revenues	84	77	262	284
Total operating revenues	2,374	2,457	7,245	7,353
Claims, Benefits and Expenses
Net incurred claims and benefits	1,351	1,374	4,231	4,249
Policyholders' dividends	3	4	10	10
Amortization of deferred acquisition costs	332	341	996	1,004
Other insurance related expenses	260	252	781	765
Other expenses	172	116	341	345
Total claims, benefits and expenses	2,118	2,087	6,359	6,373
Operating income (loss) before income tax	256	370	886	980
Income tax (expense) benefit on operating income (loss)	(74)	(99)	(242)	(285)
Net operating income (loss)	182	271	644	695
Net realized investment gains (losses), pre-tax	37	1	69	10
Income tax (expense) benefit on net realized investment gains (losses)	(10)	(1)	(23)	(3)
Net realized investment gains (losses)	27		46	7
Income (loss) from continuing operations	209	271	690	702
CNA Specialty [Member]
Operating Revenues
Net earned premiums	775	768	2,286	2,237
Net investment income	133	159	445	480
Other revenues	76	67	218	190
Total operating revenues	984	994	2,949	2,907
Claims, Benefits and Expenses
Net incurred claims and benefits	398	427	1,304	1,344
Policyholders' dividends	2	3	4	5
Amortization of deferred acquisition costs	159	161	470	467
Other insurance related expenses	73	64	219	200
Other expenses	65	58	191	172
Total claims, benefits and expenses	697	713	2,188	2,188
Operating income (loss) before income tax	287	281	761	719
Income tax (expense) benefit on operating income (loss)	(95)	(94)	(253)	(244)
Net operating income (loss)	192	187	508	475
Net realized investment gains (losses), pre-tax	3	2	10	(1)
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(1)	(3)
Net realized investment gains (losses)	2	1	7	(1)
Income (loss) from continuing operations	194	188	515	474
CNA Commercial [Member]
Operating Revenues
Net earned premiums	808	832	2,447	2,509
Net investment income	163	219	569	680
Other revenues	11	10	29	85
Total operating revenues	982	1,061	3,045	3,274
Claims, Benefits and Expenses
Net incurred claims and benefits	595	565	1,871	1,809
Policyholders' dividends	1	1	6	5
Amortization of deferred acquisition costs	139	148	423	450
Other insurance related expenses	139	141	416	426
Other expenses	12	7	31	18
Total claims, benefits and expenses	886	862	2,747	2,708
Operating income (loss) before income tax	96	199	298	566
Income tax (expense) benefit on operating income (loss)	(32)	(68)	(97)	(198)
Net operating income (loss)	64	131	201	368
Net realized investment gains (losses), pre-tax	3	1	8	(7)
Income tax (expense) benefit on net realized investment gains (losses)	1		(2)	3
Net realized investment gains (losses)	4	1	6	(4)
Income (loss) from continuing operations	68	132	207	364
Hardy [Member]
Operating Revenues
Net earned premiums	88	86	278	226
Net investment income	2	1	4	3
Other revenues	(1)	1	(1)	1
Total operating revenues	89	88	281	230
Claims, Benefits and Expenses
Net incurred claims and benefits	57	35	147	107
Policyholders' dividends
Amortization of deferred acquisition costs	27	25	81	66
Other insurance related expenses	15	14	51	43
Other expenses	6	2	14	15
Total claims, benefits and expenses	105	76	293	231
Operating income (loss) before income tax	(16)	12	(12)	(1)
Income tax (expense) benefit on operating income (loss)	1		(1)	3
Net operating income (loss)	(15)	12	(13)	2
Net realized investment gains (losses), pre-tax			(1)	2
Income tax (expense) benefit on net realized investment gains (losses)		1
Net realized investment gains (losses)		1	(1)	2
Income (loss) from continuing operations	(15)	13	(14)	4
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	139	140	417	419
Net investment income	177	167	521	492
Other revenues	(3)	(4)	10	1
Total operating revenues	313	303	948	912
Claims, Benefits and Expenses
Net incurred claims and benefits	319	343	929	983
Policyholders' dividends
Amortization of deferred acquisition costs	7	7	22	21
Other insurance related expenses	33	33	96	97
Other expenses	36		34	7
Total claims, benefits and expenses	395	383	1,081	1,108
Operating income (loss) before income tax	(82)	(80)	(133)	(196)
Income tax (expense) benefit on operating income (loss)	40	47	98	116
Net operating income (loss)	(42)	(33)	(35)	(80)
Net realized investment gains (losses), pre-tax	30	(3)	42	6
Income tax (expense) benefit on net realized investment gains (losses)	(9)		(14)	(2)
Net realized investment gains (losses)	21	(3)	28	4
Income (loss) from continuing operations	(21)	(36)	(7)	(76)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums
Net investment income	5	9	17	25
Other revenues	3	3	10	8
Total operating revenues	8	12	27	33
Claims, Benefits and Expenses
Net incurred claims and benefits	(18)	4	(20)	6
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses		1		1
Other expenses	55	49	75	134
Total claims, benefits and expenses	37	54	55	141
Operating income (loss) before income tax	(29)	(42)	(28)	(108)
Income tax (expense) benefit on operating income (loss)	12	16	11	38
Net operating income (loss)	(17)	(26)	(17)	(70)
Net realized investment gains (losses), pre-tax	1	1	10	10
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(1)	(4)	(4)
Net realized investment gains (losses)			6	6
Income (loss) from continuing operations	(17)	(26)	(11)	(64)
Eliminations [Member]
Operating Revenues
Net earned premiums		(1)	(1)	(2)
Net investment income
Other revenues	(2)		(4)	(1)
Total operating revenues	(2)	(1)	(5)	(3)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses		(1)	(1)	(2)
Other expenses	(2)		(4)	(1)
Total claims, benefits and expenses	(2)	(1)	(5)	(3)
Operating income (loss) before income tax
Income tax (expense) benefit on operating income (loss)
Net operating income (loss)
Net realized investment gains (losses), pre-tax
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses)
Income (loss) from continuing operations