Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Operating Revenues
Net earned premiums	$ 1,781	$ 1,732	$ 5,098	$ 4,942
Net investment income	601	394	1,719	1,531
Other revenues	76	76	230	214
Total operating revenues	2,458	2,202	7,047	6,687
Claims, Benefits and Expenses
Net incurred claims and benefits	1,430	1,399	4,150	4,125
Policyholders' dividends	5	1	14	6
Amortization of deferred acquisition costs	333	297	937	880
Other insurance related expenses	268	252	766	726
Other expenses	116	102	338	320
Total claims, benefits and expenses	2,152	2,051	6,205	6,057
Operating income (loss) from continuing operations before income tax	306	151	842	630
Income tax (expense) benefit on operating income (loss)	(90)	(58)	(248)	(197)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(2)		(15)
Net operating income (loss) from continuing operations attributable to CNA	216	91	594	418
Net realized investment gains (losses), net of participating policyholders’ interests	8	(27)	66	1
Income tax (expense) benefit on net realized investment gains (losses)	(3)	10	(23)	1
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests		1
Net realized investment gains (losses) attributable to CNA	5	(16)	43	2
Net income (loss) from continuing operations attributable to CNA	221	75	637	420
CNA Specialty [Member]
Operating Revenues
Net earned premiums	738	741	2,163	2,098
Net investment income	159	85	446	371
Other revenues	58	56	171	164
Total operating revenues	955	882	2,780	2,633
Claims, Benefits and Expenses
Net incurred claims and benefits	460	485	1,376	1,333
Policyholders' dividends	1	(4)		(4)
Amortization of deferred acquisition costs	156	147	458	428
Other insurance related expenses	73	75	222	216
Other expenses	53	48	153	140
Total claims, benefits and expenses	743	751	2,209	2,113
Operating income (loss) from continuing operations before income tax	212	131	571	520
Income tax (expense) benefit on operating income (loss)	(76)	(48)	(197)	(182)
Net operating (income) loss, after-tax, attributable to noncontrolling interests				(12)
Net operating income (loss) from continuing operations attributable to CNA	136	83	374	326
Net realized investment gains (losses), net of participating policyholders’ interests	2	(8)	18	7
Income tax (expense) benefit on net realized investment gains (losses)	(2)	3	(6)	(2)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA		(5)	12	5
Net income (loss) from continuing operations attributable to CNA	136	78	386	331
CNA Commercial [Member]
Operating Revenues
Net earned premiums	840	848	2,452	2,418
Net investment income	230	115	646	569
Other revenues	11	14	31	40
Total operating revenues	1,081	977	3,129	3,027
Claims, Benefits and Expenses
Net incurred claims and benefits	591	583	1,749	1,807
Policyholders' dividends	3	3	9	5
Amortization of deferred acquisition costs	151	145	437	435
Other insurance related expenses	146	141	425	400
Other expenses	7	10	24	37
Total claims, benefits and expenses	898	882	2,644	2,684
Operating income (loss) from continuing operations before income tax	183	95	485	343
Income tax (expense) benefit on operating income (loss)	(58)	(46)	(164)	(127)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(2)		(3)
Net operating income (loss) from continuing operations attributable to CNA	125	47	321	213
Net realized investment gains (losses), net of participating policyholders’ interests	10	(15)	34	13
Income tax (expense) benefit on net realized investment gains (losses)	(3)	5	(12)	(4)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests		1
Net realized investment gains (losses) attributable to CNA	7	(9)	22	9
Net income (loss) from continuing operations attributable to CNA	132	38	343	222
Hardy [Member]
Operating Revenues
Net earned premiums	64		64
Net investment income	2		2
Other revenues
Total operating revenues	66		66
Claims, Benefits and Expenses
Net incurred claims and benefits	21		21
Policyholders' dividends
Amortization of deferred acquisition costs	20		20
Other insurance related expenses	13		13
Other expenses	7		7
Total claims, benefits and expenses	61		61
Operating income (loss) from continuing operations before income tax	5		5
Income tax (expense) benefit on operating income (loss)	(2)		(2)
Net operating income (loss) from continuing operations attributable to CNA	3		3
Net realized investment gains (losses), net of participating policyholders’ interests	(1)		(1)
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses) attributable to CNA	(1)		(1)
Net income (loss) from continuing operations attributable to CNA	2		2
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	141	142	421	427
Net investment income	201	190	600	567
Other revenues	1	6	15	6
Total operating revenues	343	338	1,036	1,000
Claims, Benefits and Expenses
Net incurred claims and benefits	350	332	1,009	985
Policyholders' dividends	1	2	5	5
Amortization of deferred acquisition costs	6	5	22	17
Other insurance related expenses	35	36	106	108
Other expenses	8	2	18	14
Total claims, benefits and expenses	400	377	1,160	1,129
Operating income (loss) from continuing operations before income tax	(57)	(39)	(124)	(129)
Income tax (expense) benefit on operating income (loss)	35	25	86	78
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(22)	(14)	(38)	(51)
Net realized investment gains (losses), net of participating policyholders’ interests	(3)	(4)	14	(7)
Income tax (expense) benefit on net realized investment gains (losses)	1	1	(5)	2
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	(2)	(3)	9	(5)
Net income (loss) from continuing operations attributable to CNA	(24)	(17)	(29)	(56)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums	(2)	2	(1)	1
Net investment income	9	4	25	24
Other revenues	7		14	4
Total operating revenues	14	6	38	29
Claims, Benefits and Expenses
Net incurred claims and benefits	8	(1)	(5)
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	1	1	1	4
Other expenses	42	42	137	129
Total claims, benefits and expenses	51	42	133	133
Operating income (loss) from continuing operations before income tax	(37)	(36)	(95)	(104)
Income tax (expense) benefit on operating income (loss)	11	11	29	34
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(26)	(25)	(66)	(70)
Net realized investment gains (losses), net of participating policyholders’ interests			1	(12)
Income tax (expense) benefit on net realized investment gains (losses)	1	1		5
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	1	1	1	(7)
Net income (loss) from continuing operations attributable to CNA	(25)	(24)	(65)	(77)
Eliminations [Member]
Operating Revenues
Net earned premiums		(1)	(1)	(2)
Net investment income
Other revenues	(1)		(1)
Total operating revenues	(1)	(1)	(2)	(2)
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	(1)		(1)
Total claims, benefits and expenses	(1)	(1)	(2)	(2)
Operating income (loss) from continuing operations before income tax
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