Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating Revenues
Net earned premiums	$ 1,649	$ 1,615
Net investment income	648	620
Other revenues	68	67
Total operating revenues	2,365	2,302
Claims, Benefits and Expenses
Net incurred claims and benefits	1,378	1,363
Policyholders' dividends	3	1
Amortization of deferred acquisition costs	295	297
Other insurance related expenses	250	217
Other expenses	111	106
Total claims, benefits and expenses	2,037	1,984
Operating income (loss) from continuing operations before income tax	328	318
Income tax (expense) benefit on operating income (loss)	(102)	(97)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(8)
Net operating income (loss) from continuing operations attributable to CNA	226	213
Net realized investment gains (losses), net of participating policyholders’ interests	36	13
Income tax (expense) benefit on net realized investment gains (losses)	(12)	(4)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests		(1)
Net realized investment gains (losses) attributable to CNA	24	8
Net income (loss) from continuing operations attributable to CNA	250	221
CNA Specialty [Member]
Operating Revenues
Net earned premiums	706	669
Net investment income	175	160
Other revenues	56	54
Total operating revenues	937	883
Claims, Benefits and Expenses
Net incurred claims and benefits	468	430
Policyholders' dividends	(2)
Amortization of deferred acquisition costs	148	143
Other insurance related expenses	72	64
Other expenses	50	40
Total claims, benefits and expenses	736	677
Operating income (loss) from continuing operations before income tax	201	206
Income tax (expense) benefit on operating income (loss)	(69)	(70)
Net operating (income) loss, after-tax, attributable to noncontrolling interests		(8)
Net operating income (loss) from continuing operations attributable to CNA	132	128
Net realized investment gains (losses), net of participating policyholders’ interests	8	8
Income tax (expense) benefit on net realized investment gains (losses)	(2)	(3)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	6	5
Net income (loss) from continuing operations attributable to CNA	138	133
CNA Commercial [Member]
Operating Revenues
Net earned premiums	803	802
Net investment income	265	261
Other revenues	9	14
Total operating revenues	1,077	1,077
Claims, Benefits and Expenses
Net incurred claims and benefits	567	603
Policyholders' dividends	3
Amortization of deferred acquisition costs	139	148
Other insurance related expenses	144	115
Other expenses	7	16
Total claims, benefits and expenses	860	882
Operating income (loss) from continuing operations before income tax	217	195
Income tax (expense) benefit on operating income (loss)	(78)	(65)
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	139	130
Net realized investment gains (losses), net of participating policyholders’ interests	11	17
Income tax (expense) benefit on net realized investment gains (losses)	(4)	(6)
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests		(1)
Net realized investment gains (losses) attributable to CNA	7	10
Net income (loss) from continuing operations attributable to CNA	146	140
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	141	144
Net investment income	198	188
Other revenues	(2)	(2)
Total operating revenues	337	330
Claims, Benefits and Expenses
Net incurred claims and benefits	336	323
Policyholders' dividends	2	1
Amortization of deferred acquisition costs	8	6
Other insurance related expenses	35	38
Other expenses	6	6
Total claims, benefits and expenses	387	374
Operating income (loss) from continuing operations before income tax	(50)	(44)
Income tax (expense) benefit on operating income (loss)	31	26
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(19)	(18)
Net realized investment gains (losses), net of participating policyholders’ interests	13	(4)
Income tax (expense) benefit on net realized investment gains (losses)	(5)	1
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	8	(3)
Net income (loss) from continuing operations attributable to CNA	(11)	(21)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums	(1)	1
Net investment income	10	11
Other revenues	5	1
Total operating revenues	14	13
Claims, Benefits and Expenses
Net incurred claims and benefits	7	7
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	(1)	1
Other expenses	48	44
Total claims, benefits and expenses	54	52
Operating income (loss) from continuing operations before income tax	(40)	(39)
Income tax (expense) benefit on operating income (loss)	14	12
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(26)	(27)
Net realized investment gains (losses), net of participating policyholders’ interests	4	(8)
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)
Net income (loss) from continuing operations attributable to CNA	(23)	(31)
Eliminations [Member]
Operating Revenues
Net earned premiums		(1)
Net investment income
Other revenues
Total operating revenues		(1)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses		(1)
Other expenses
Total claims, benefits and expenses		(1)
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