Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Net written premiums	$ 6,798	[1]	$ 6,471	[1]	$ 6,713	[1]
Operating Revenues
Net earned premiums	6,603		6,515		6,721
Net investment income	2,054		2,316		2,320
Other revenues	294		292		288
Total operating revenues	8,951		9,123		9,329
Claims, Benefits and Expenses
Net incurred claims and benefits	5,476		4,955		5,267
Policyholders' dividends	13		30		23
Amortization of deferred acquisition costs	1,410		1,387		1,417
Other insurance related expenses	738		797		781
Other expenses	433		928		444
Total claims, benefits and expenses	8,070		8,097		7,932
Operating income (loss) from continuing operations before income tax	881		1,026		1,397
Income tax (expense) benefit on operating income (loss)	(251)		(297)		(353)
Net operating (income) loss, after-tax, attributable to noncontrolling interests	(16)		(69)		(62)
Net operating income (loss) from continuing operations attributable to CNA	614		660		982
Net realized investment gains (losses), net of participating policyholders’ interests	(4)		86		(857)
Income tax (expense) benefit on net realized investment gains (losses)	5		(36)		296
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests			1
Net realized investment gains (losses) attributable to CNA	1		51		(561)
Net income (loss) from continuing operations attributable to CNA	615		711		421
CNA Specialty [Member]
Net written premiums	2,872	[1]	2,691	[1]	2,684	[1]
Operating Revenues
Net earned premiums	2,796		2,679		2,697
Net investment income	500		591		526
Other revenues	221		216		206
Total operating revenues	3,517		3,486		3,429
Claims, Benefits and Expenses
Net incurred claims and benefits	1,657		1,447		1,536
Policyholders' dividends	(3)		12		9
Amortization of deferred acquisition costs	663		631		624
Other insurance related expenses	197		186		163
Other expenses	191		190		179
Total claims, benefits and expenses	2,705		2,466		2,511
Operating income (loss) from continuing operations before income tax	812		1,020		918
Income tax (expense) benefit on operating income (loss)	(281)		(343)		(282)
Net operating (income) loss, after-tax, attributable to noncontrolling interests	(12)		(52)		(45)
Net operating income (loss) from continuing operations attributable to CNA	519		625		591
Net realized investment gains (losses), net of participating policyholders’ interests	(5)		30		(186)
Income tax (expense) benefit on net realized investment gains (losses)	2		(10)		64
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests					(1)
Net realized investment gains (losses) attributable to CNA	(3)		20		(123)
Net income (loss) from continuing operations attributable to CNA	516		645		468
CNA Commercial [Member]
Net written premiums	3,350	[1]	3,208	[1]	3,448	[1]
Operating Revenues
Net earned premiums	3,240		3,256		3,432
Net investment income	763		873		935
Other revenues	54		61		61
Total operating revenues	4,057		4,190		4,428
Claims, Benefits and Expenses
Net incurred claims and benefits	2,296		2,175		2,420
Policyholders' dividends	8		14		9
Amortization of deferred acquisition costs	725		736		775
Other insurance related expenses	395		424		435
Other expenses	53		55		77
Total claims, benefits and expenses	3,477		3,404		3,716
Operating income (loss) from continuing operations before income tax	580		786		712
Income tax (expense) benefit on operating income (loss)	(207)		(260)		(201)
Net operating (income) loss, after-tax, attributable to noncontrolling interests	(4)		(17)		(17)
Net operating income (loss) from continuing operations attributable to CNA	369		509		494
Net realized investment gains (losses), net of participating policyholders’ interests	14		(15)		(360)
Income tax (expense) benefit on net realized investment gains (losses)	(2)		(1)		123
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests			1		1
Net realized investment gains (losses) attributable to CNA	12		(15)		(236)
Net income (loss) from continuing operations attributable to CNA	381		494		258
Life and Group Non-Core [Member]
Net written premiums	577	[1]	573	[1]	583	[1]
Operating Revenues
Net earned premiums	569		582		595
Net investment income	759		715		664
Other revenues	13		7		11
Total operating revenues	1,341		1,304		1,270
Claims, Benefits and Expenses
Net incurred claims and benefits	1,526		1,275		1,084
Policyholders' dividends	8		4		5
Amortization of deferred acquisition costs	22		20		18
Other insurance related expenses	143		180		183
Other expenses	19		2		69
Total claims, benefits and expenses	1,718		1,481		1,359
Operating income (loss) from continuing operations before income tax	(377)		(177)		(89)
Income tax (expense) benefit on operating income (loss)	169		90		73
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(208)		(87)		(16)
Net realized investment gains (losses), net of participating policyholders’ interests	(7)		53		(235)
Income tax (expense) benefit on net realized investment gains (losses)	2		(20)		82
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	(5)		33		(153)
Net income (loss) from continuing operations attributable to CNA	(213)		(54)		(169)
Corporate and Other Non-Core [Member]
Net written premiums	2	[1]	2	[1]	1	[1]
Operating Revenues
Net earned premiums	1		1
Net investment income	32		137		195
Other revenues	6		8		10
Total operating revenues	39		146		205
Claims, Benefits and Expenses
Net incurred claims and benefits	(3)		58		227
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	6		10		3
Other expenses	170		681		119
Total claims, benefits and expenses	173		749		349
Operating income (loss) from continuing operations before income tax	(134)		(603)		(144)
Income tax (expense) benefit on operating income (loss)	68		216		57
Net operating (income) loss, after-tax, attributable to noncontrolling interests
Net operating income (loss) from continuing operations attributable to CNA	(66)		(387)		(87)
Net realized investment gains (losses), net of participating policyholders’ interests	(6)		18		(76)
Income tax (expense) benefit on net realized investment gains (losses)	3		(5)		27
Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests
Net realized investment gains (losses) attributable to CNA	(3)		13		(49)
Net income (loss) from continuing operations attributable to CNA	(69)		(374)		(136)
Eliminations [Member]
Net written premiums	(3)	[1]	(3)	[1]	(3)	[1]
Operating Revenues
Net earned premiums	(3)		(3)		(3)
Net investment income
Other revenues
Total operating revenues	(3)		(3)		(3)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	(3)		(3)		(3)
Other expenses
Total claims, benefits and expenses	(3)		(3)		(3)
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

[1] Related to business in property and casualty companies only.