Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Operating Revenues
Net earned premiums	$ 1,668	$ 1,595	$ 3,317	$ 3,210
Net investment income	470	517	1,118	1,137
Other revenues	86	71	154	138
Total operating revenues	2,224	2,183	4,589	4,485
Claims, Benefits and Expenses
Net incurred claims and benefits	1,342	1,363	2,720	2,726
Policyholders' dividends	6	4	9	5
Amortization of deferred acquisition costs	309	286	604	583
Other insurance related expenses	248	257	498	474
Other expenses	111	112	222	218
Total claims, benefits and expenses	2,016	2,022	4,053	4,006
Operating income (loss) from continuing operations before income tax	208	161	536	479
Income tax (expense) benefit on operating income (loss)	(56)	(42)	(158)	(139)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(5)		(13)
Net operating income (loss) from continuing operations attributable to CNA	152	114	378	327
Net realized investment gains (losses), net of participating policyholders’ interests	22	15	58	28
Income tax (expense) benefit on net realized investment gains (losses)	(8)	(5)	(20)	(9)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests				(1)
Net realized investment gains (losses) attributable to CNA	14	10	38	18
Net income (loss) from continuing operations attributable to CNA	166	124	416	345
CNA Specialty [Member]
Operating Revenues
Net earned premiums	719	688	1,425	1,357
Net investment income	112	126	287	286
Other revenues	57	54	113	108
Total operating revenues	888	868	1,825	1,751
Claims, Benefits and Expenses
Net incurred claims and benefits	448	418	916	848
Policyholders' dividends	1		(1)
Amortization of deferred acquisition costs	154	138	302	281
Other insurance related expenses	77	77	149	141
Other expenses	50	52	100	92
Total claims, benefits and expenses	730	685	1,466	1,362
Operating income (loss) from continuing operations before income tax	158	183	359	389
Income tax (expense) benefit on operating income (loss)	(52)	(64)	(121)	(134)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(4)		(12)
Net operating income (loss) from continuing operations attributable to CNA	106	115	238	243
Net realized investment gains (losses), net of participating policyholders’ interests	8	7	16	15
Income tax (expense) benefit on net realized investment gains (losses)	(2)	(2)	(4)	(5)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	6	5	12	10
Net income (loss) from continuing operations attributable to CNA	112	120	250	253
CNA Commercial [Member]
Operating Revenues
Net earned premiums	809	768	1,612	1,570
Net investment income	151	193	416	454
Other revenues	11	12	20	26
Total operating revenues	971	973	2,048	2,050
Claims, Benefits and Expenses
Net incurred claims and benefits	591	621	1,158	1,224
Policyholders' dividends	3	2	6	2
Amortization of deferred acquisition costs	147	142	286	290
Other insurance related expenses	135	144	279	259
Other expenses	10	11	17	27
Total claims, benefits and expenses	886	920	1,746	1,802
Operating income (loss) from continuing operations before income tax	85	53	302	248
Income tax (expense) benefit on operating income (loss)	(28)	(16)	(106)	(81)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(1)		(1)
Net operating income (loss) from continuing operations attributable to CNA	57	36	196	166
Net realized investment gains (losses), net of participating policyholders’ interests	13	11	24	28
Income tax (expense) benefit on net realized investment gains (losses)	(5)	(3)	(9)	(9)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests				(1)
Net realized investment gains (losses) attributable to CNA	8	8	15	18
Net income (loss) from continuing operations attributable to CNA	65	44	211	184
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	139	141	280	285
Net investment income	201	189	399	377
Other revenues	16	2	14
Total operating revenues	356	332	693	662
Claims, Benefits and Expenses
Net incurred claims and benefits	323	330	659	653
Policyholders' dividends	2	2	4	3
Amortization of deferred acquisition costs	8	6	16	12
Other insurance related expenses	36	34	71	72
Other expenses	4	6	10	12
Total claims, benefits and expenses	373	378	760	752
Operating income (loss) from continuing operations before income tax	(17)	(46)	(67)	(90)
Income tax (expense) benefit on operating income (loss)	20	27	51	53
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	3	(19)	(16)	(37)
Net realized investment gains (losses), net of participating policyholders’ interests	4	1	17	(3)
Income tax (expense) benefit on net realized investment gains (losses)	(1)		(6)	1
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	3	1	11	(2)
Net income (loss) from continuing operations attributable to CNA	6	(18)	(5)	(39)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums	2	(2)	1	(1)
Net investment income	6	9	16	20
Other revenues	2	3	7	4
Total operating revenues	10	10	24	23
Claims, Benefits and Expenses
Net incurred claims and benefits	(20)	(6)	(13)	1
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	1	2		3
Other expenses	47	43	95	87
Total claims, benefits and expenses	28	39	82	91
Operating income (loss) from continuing operations before income tax	(18)	(29)	(58)	(68)
Income tax (expense) benefit on operating income (loss)	4	11	18	23
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(14)	(18)	(40)	(45)
Net realized investment gains (losses), net of participating policyholders’ interests	(3)	(4)	1	(12)
Income tax (expense) benefit on net realized investment gains (losses)			(1)	4
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	(3)	(4)		(8)
Net income (loss) from continuing operations attributable to CNA	(17)	(22)	(40)	(53)
Eliminations [Member]
Operating Revenues
Net earned premiums	(1)		(1)	(1)
Net investment income
Other revenues
Total operating revenues	(1)		(1)	(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
Total claims, benefits and expenses	(1)		(1)	(1)
Operating income (loss) from continuing operations before income tax			0
Income tax (expense) benefit on operating income (loss)
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA
Net realized investment gains (losses), net of participating policyholders’ interests
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA
Net income (loss) from continuing operations attributable to CNA