Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Net written premiums	$ 7,348	[1]	$ 6,964	[1]	$ 6,798	[1]
Operating Revenues
Net earned premiums	7,271		6,882		6,603
Net investment income	2,450		2,282		2,054
Other revenues	361		320		294
Total operating revenues	10,082		9,484		8,951
Claims, Benefits and Expenses
Net incurred claims and benefits	5,927		5,867		5,476
Policyholders' dividends	20		29		13
Amortization of deferred acquisition costs	1,362		1,274		1,176
Other insurance related expenses	1,017		1,049		980
Other expenses	474		456		433
Total claims, benefits and expenses	8,800		8,675		8,078
Operating income (loss) from continuing operations before income tax	1,282		809		873
Income tax (expense) benefit on operating income (loss)	(365)		(222)		(247)
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests					(16)
Net operating income (loss) from continuing operations attributable to CNA	917		587		610
Net realized investment gains (losses), pre-tax	31		63		(2)
Income tax (expense) benefit on net realized investment gains (losses)	(11)		(22)		5
Net realized investment gains (losses), after tax	20		41		3
Net income (loss) from continuing operations attributable to CNA	937		628		613
CNA Specialty [Member]
Segment Reporting Information [Line Items]
Net written premiums	3,091	[1]	2,924	[1]	2,872	[1]
Operating Revenues
Net earned premiums	3,004		2,898		2,796
Net investment income	657		592		500
Other revenues	257		230		221
Total operating revenues	3,918		3,720		3,517
Claims, Benefits and Expenses
Net incurred claims and benefits	1,704		1,831		1,657
Policyholders' dividends	6		2		(3)
Amortization of deferred acquisition costs	628		614		568
Other insurance related expenses	272		301		294
Other expenses	236		206		191
Total claims, benefits and expenses	2,846		2,954		2,707
Operating income (loss) from continuing operations before income tax	1,072		766		810
Income tax (expense) benefit on operating income (loss)	(365)		(262)		(281)
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests					(12)
Net operating income (loss) from continuing operations attributable to CNA	707		504		517
Net realized investment gains (losses), pre-tax	(3)		22		(5)
Income tax (expense) benefit on net realized investment gains (losses)	1		(9)		2
Net realized investment gains (losses), after tax	(2)		13		(3)
Net income (loss) from continuing operations attributable to CNA	705		517		514
CNA Commercial [Member]
Segment Reporting Information [Line Items]
Net written premiums	3,312	[1]	3,373	[1]	3,350	[1]
Operating Revenues
Net earned premiums	3,350		3,306		3,240
Net investment income	927		854		763
Other revenues	96		40		54
Total operating revenues	4,373		4,200		4,057
Claims, Benefits and Expenses
Net incurred claims and benefits	2,475		2,574		2,296
Policyholders' dividends	7		11		8
Amortization of deferred acquisition costs	600		588		585
Other insurance related expenses	547		578		540
Other expenses	26		36		53
Total claims, benefits and expenses	3,655		3,787		3,482
Operating income (loss) from continuing operations before income tax	718		413		575
Income tax (expense) benefit on operating income (loss)	(250)		(136)		(204)
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests					(4)
Net operating income (loss) from continuing operations attributable to CNA	468		277		367
Net realized investment gains (losses), pre-tax	(13)		38		16
Income tax (expense) benefit on net realized investment gains (losses)	4		(11)		(2)
Net realized investment gains (losses), after tax	(9)		27		14
Net income (loss) from continuing operations attributable to CNA	459		304		381
Hardy [Member]
Segment Reporting Information [Line Items]
Net written premiums	396	[1]	117	[1],[2]
Operating Revenues
Net earned premiums	361		120	[2]
Net investment income	4		3	[2]
Other revenues			1	[2]
Total operating revenues	365		124	[2]
Claims, Benefits and Expenses
Net incurred claims and benefits	162		72	[2]
Policyholders' dividends				[2]
Amortization of deferred acquisition costs	106		44	[2]
Other insurance related expenses	69		25	[2]
Other expenses	19		9	[2]
Total claims, benefits and expenses	356		150	[2]
Operating income (loss) from continuing operations before income tax	9		(26)	[2]
Income tax (expense) benefit on operating income (loss)	1		3	[2]
Net operating income (loss) from continuing operations attributable to CNA	10		(23)	[2]
Net realized investment gains (losses), pre-tax	1		(1)	[2]
Income tax (expense) benefit on net realized investment gains (losses)				[2]
Net realized investment gains (losses), after tax	1		(1)	[2]
Net income (loss) from continuing operations attributable to CNA	11		(24)	[2]
Life and Group Non-Core [Member]
Segment Reporting Information [Line Items]
Net written premiums	552	[1]	553	[1]	577	[1]
Operating Revenues
Net earned premiums	559		560		569
Net investment income	830		801		759
Other revenues	(2)		34		13
Total operating revenues	1,387		1,395		1,341
Claims, Benefits and Expenses
Net incurred claims and benefits	1,395		1,406		1,526
Policyholders' dividends	7		16		8
Amortization of deferred acquisition costs	28		28		23
Other insurance related expenses	134		144		143
Other expenses	12		23		19
Total claims, benefits and expenses	1,576		1,617		1,719
Operating income (loss) from continuing operations before income tax	(189)		(222)		(378)
Income tax (expense) benefit on operating income (loss)	131		132		170
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests
Net operating income (loss) from continuing operations attributable to CNA	(58)		(90)		(208)
Net realized investment gains (losses), pre-tax	37				(7)
Income tax (expense) benefit on net realized investment gains (losses)	(13)				2
Net realized investment gains (losses), after tax	24				(5)
Net income (loss) from continuing operations attributable to CNA	(34)		(90)		(213)
Corporate and Other Non-Core [Member]
Segment Reporting Information [Line Items]
Net written premiums	(1)	[1]	(1)	[1]	2	[1]
Operating Revenues
Net earned premiums	(1)				1
Net investment income	32		32		32
Other revenues	12		16		6
Total operating revenues	43		48		39
Claims, Benefits and Expenses
Net incurred claims and benefits	191		(16)		(3)
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	(3)		3		6
Other expenses	183		183		170
Total claims, benefits and expenses	371		170		173
Operating income (loss) from continuing operations before income tax	(328)		(122)		(134)
Income tax (expense) benefit on operating income (loss)	118		41		68
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests
Net operating income (loss) from continuing operations attributable to CNA	(210)		(81)		(66)
Net realized investment gains (losses), pre-tax	9		4		(6)
Income tax (expense) benefit on net realized investment gains (losses)	(3)		(2)		3
Net realized investment gains (losses), after tax	6		2		(3)
Net income (loss) from continuing operations attributable to CNA	(204)		(79)		(69)
Eliminations [Member]
Segment Reporting Information [Line Items]
Net written premiums	(2)	[1]	(2)	[1]	(3)	[1]
Operating Revenues
Net earned premiums	(2)		(2)		(3)
Net investment income
Other revenues	(2)		(1)
Total operating revenues	(4)		(3)		(3)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	(2)		(2)		(3)
Other expenses	(2)		(1)
Total claims, benefits and expenses	(4)		(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), pre-tax
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses), after tax
Net income (loss) from continuing operations attributable to CNA

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

[2] (b)Included from date of acquisition.