Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Operating Revenues
Net earned premiums	$ 1,800	$ 1,668	$ 3,564	$ 3,317
Net investment income	578	470	1,211	1,118
Other revenues	129	86	207	154
Total operating revenues	2,507	2,224	4,982	4,589
Claims, Benefits and Expenses
Net incurred claims and benefits	1,515	1,342	2,940	2,720
Policyholders' dividends	6	6	10	9
Amortization of deferred acquisition costs	335	309	663	604
Other insurance related expenses	253	248	515	498
Other expenses	108	111	229	222
Total claims, benefits and expenses	2,217	2,016	4,357	4,053
Operating income (loss) before income tax	290	208	625	536
Income tax (expense) benefit on operating income (loss)	(86)	(56)	(190)	(158)
Net operating income (loss)	204	152	435	378
Net realized investment gains (losses), pre-tax	(14)	22	14	58
Income tax (expense) benefit on net realized investment gains (losses)	4	(8)	(5)	(20)
Net realized investment gains (losses)	(10)	14	9	38
Net income (loss)	194	166	444	416
CNA Specialty [Member]
Operating Revenues
Net earned premiums	746	719	1,469	1,425
Net investment income	151	112	321	287
Other revenues	63	57	123	113
Total operating revenues	960	888	1,913	1,825
Claims, Benefits and Expenses
Net incurred claims and benefits	452	448	917	916
Policyholders' dividends	1	1	2	(1)
Amortization of deferred acquisition costs	156	154	306	302
Other insurance related expenses	65	77	136	149
Other expenses	60	50	114	100
Total claims, benefits and expenses	734	730	1,475	1,466
Operating income (loss) before income tax	226	158	438	359
Income tax (expense) benefit on operating income (loss)	(78)	(52)	(150)	(121)
Net operating income (loss)	148	106	288	238
Net realized investment gains (losses), pre-tax	(6)	8	(3)	16
Income tax (expense) benefit on net realized investment gains (losses)	2	(2)	1	(4)
Net realized investment gains (losses)	(4)	6	(2)	12
Net income (loss)	144	112	286	250
CNA Commercial [Member]
Operating Revenues
Net earned premiums	840	809	1,677	1,612
Net investment income	211	151	461	416
Other revenues	65	11	75	20
Total operating revenues	1,116	971	2,213	2,048
Claims, Benefits and Expenses
Net incurred claims and benefits	644	591	1,244	1,158
Policyholders' dividends	2	3	4	6
Amortization of deferred acquisition costs	153	147	302	286
Other insurance related expenses	142	135	285	279
Other expenses	2	10	11	17
Total claims, benefits and expenses	943	886	1,846	1,746
Operating income (loss) before income tax	173	85	367	302
Income tax (expense) benefit on operating income (loss)	(61)	(28)	(130)	(106)
Net operating income (loss)	112	57	237	196
Net realized investment gains (losses), pre-tax	(12)	13	(8)	24
Income tax (expense) benefit on net realized investment gains (losses)	4	(5)	3	(9)
Net realized investment gains (losses)	(8)	8	(5)	15
Net income (loss)	104	65	232	211
Hardy [Member]
Operating Revenues
Net earned premiums	76		140
Net investment income	1		2
Other revenues	(1)
Total operating revenues	76		142
Claims, Benefits and Expenses
Net incurred claims and benefits	41		72
Policyholders' dividends
Amortization of deferred acquisition costs	20		41
Other insurance related expenses	13		29
Other expenses	6		13
Total claims, benefits and expenses	80		155
Operating income (loss) before income tax	(4)		(13)
Income tax (expense) benefit on operating income (loss)	2		3
Net operating income (loss)	(2)		(10)
Net realized investment gains (losses), pre-tax	1		2
Income tax (expense) benefit on net realized investment gains (losses)	(1)		(1)
Net realized investment gains (losses)			1
Net income (loss)	(2)		(9)
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	138	139	279	280
Net investment income	207	201	411	399
Other revenues	1	16	5	14
Total operating revenues	346	356	695	693
Claims, Benefits and Expenses
Net incurred claims and benefits	380	323	705	659
Policyholders' dividends	3	2	4	4
Amortization of deferred acquisition costs	6	8	14	16
Other insurance related expenses	33	36	66	71
Other expenses	2	4	7	10
Total claims, benefits and expenses	424	373	796	760
Operating income (loss) before income tax	(78)	(17)	(101)	(67)
Income tax (expense) benefit on operating income (loss)	42	20	65	51
Net operating income (loss)	(36)	3	(36)	(16)
Net realized investment gains (losses), pre-tax		4	14	17
Income tax (expense) benefit on net realized investment gains (losses)		(1)	(5)	(6)
Net realized investment gains (losses)		3	9	11
Net income (loss)	(36)	6	(27)	(5)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums		2		1
Net investment income	8	6	16	16
Other revenues	2	2	5	7
Total operating revenues	10	10	21	24
Claims, Benefits and Expenses
Net incurred claims and benefits	(2)	(20)	2	(13)
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses		1
Other expenses	39	47	85	95
Total claims, benefits and expenses	37	28	87	82
Operating income (loss) before income tax	(27)	(18)	(66)	(58)
Income tax (expense) benefit on operating income (loss)	9	4	22	18
Net operating income (loss)	(18)	(14)	(44)	(40)
Net realized investment gains (losses), pre-tax	3	(3)	9	1
Income tax (expense) benefit on net realized investment gains (losses)	(1)		(3)	(1)
Net realized investment gains (losses)	2	(3)	6
Net income (loss)	(16)	(17)	(38)	(40)
Eliminations [Member]
Operating Revenues
Net earned premiums		(1)	(1)	(1)
Net investment income
Other revenues	(1)		(1)
Total operating revenues	(1)	(1)	(2)	(1)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses		(1)	(1)	(1)
Other expenses	(1)		(1)
Total claims, benefits and expenses	(1)	(1)	(2)	(1)
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)
Net income (loss)