Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating Revenues
Net earned premiums	$ 1,806	$ 1,764
Net investment income	526	591
Other revenues	85	77
Total operating revenues	2,417	2,432
Claims, Benefits and Expenses
Net incurred claims and benefits	1,443	1,393
Policyholders' dividends	3	3
Amortization of deferred acquisition costs	329	328
Other insurance related expenses	262	261
Other expenses	128	121
Total claims, benefits and expenses	2,165	2,106
Operating income (loss) before income tax	252	326
Income tax (expense) benefit on operating income (loss)	(62)	(101)
Net operating income (loss)	190	225
Net realized investment gains (losses), pre-tax	46	23
Income tax (expense) benefit on net realized investment gains (losses)	(16)	(7)
Net realized investment gains (losses)	30	16
Net income (loss) from continuing operations	220	241
CNA Specialty [Member]
Operating Revenues
Net earned premiums	748	723
Net investment income	151	170
Other revenues	68	60
Total operating revenues	967	953
Claims, Benefits and Expenses
Net incurred claims and benefits	480	465
Policyholders' dividends	1	1
Amortization of deferred acquisition costs	153	150
Other insurance related expenses	72	71
Other expenses	61	54
Total claims, benefits and expenses	767	741
Operating income (loss) before income tax	200	212
Income tax (expense) benefit on operating income (loss)	(66)	(72)
Net operating income (loss)	134	140
Net realized investment gains (losses), pre-tax	12	3
Income tax (expense) benefit on net realized investment gains (losses)	(4)	(1)
Net realized investment gains (losses)	8	2
Net income (loss) from continuing operations	142	142
CNA Commercial [Member]
Operating Revenues
Net earned premiums	822	837
Net investment income	198	250
Other revenues	9	10
Total operating revenues	1,029	1,097
Claims, Benefits and Expenses
Net incurred claims and benefits	622	600
Policyholders' dividends	2	2
Amortization of deferred acquisition costs	140	149
Other insurance related expenses	139	143
Other expenses	11	9
Total claims, benefits and expenses	914	903
Operating income (loss) before income tax	115	194
Income tax (expense) benefit on operating income (loss)	(37)	(69)
Net operating income (loss)	78	125
Net realized investment gains (losses), pre-tax	12	4
Income tax (expense) benefit on net realized investment gains (losses)	(4)	(1)
Net realized investment gains (losses)	8	3
Net income (loss) from continuing operations	86	128
Hardy [Member]
Operating Revenues
Net earned premiums	98	64
Net investment income	1	1
Other revenues	1	1
Total operating revenues	100	66
Claims, Benefits and Expenses
Net incurred claims and benefits	38	31
Policyholders' dividends
Amortization of deferred acquisition costs	28	21
Other insurance related expenses	20	16
Other expenses	4	7
Total claims, benefits and expenses	90	75
Operating income (loss) before income tax	10	(9)
Income tax (expense) benefit on operating income (loss)	(3)	1
Net operating income (loss)	7	(8)
Net realized investment gains (losses), pre-tax		1
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses)		1
Net income (loss) from continuing operations	7	(7)
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	139	141
Net investment income	171	162
Other revenues	5	3
Total operating revenues	315	306
Claims, Benefits and Expenses
Net incurred claims and benefits	306	293
Policyholders' dividends
Amortization of deferred acquisition costs	8	8
Other insurance related expenses	32	32
Other expenses	1	5
Total claims, benefits and expenses	347	338
Operating income (loss) before income tax	(32)	(32)
Income tax (expense) benefit on operating income (loss)	30	26
Net operating income (loss)	(2)	(6)
Net realized investment gains (losses), pre-tax	16	9
Income tax (expense) benefit on net realized investment gains (losses)	(6)	(3)
Net realized investment gains (losses)	10	6
Net income (loss) from continuing operations	8
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums
Net investment income	5	8
Other revenues	2	3
Total operating revenues	7	11
Claims, Benefits and Expenses
Net incurred claims and benefits	(3)	4
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses
Other expenses	51	46
Total claims, benefits and expenses	48	50
Operating income (loss) before income tax	(41)	(39)
Income tax (expense) benefit on operating income (loss)	14	13
Net operating income (loss)	(27)	(26)
Net realized investment gains (losses), pre-tax	6	6
Income tax (expense) benefit on net realized investment gains (losses)	(2)	(2)
Net realized investment gains (losses)	4	4
Net income (loss) from continuing operations	(23)	(22)
Eliminations [Member]
Operating Revenues
Net earned premiums	(1)	(1)
Net investment income
Other revenues
Total operating revenues	(1)	(1)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	(1)	(1)
Other expenses
Total claims, benefits and expenses	(1)	(1)
Operating income (loss) before income tax	0
Income tax (expense) benefit on operating income (loss)
Net operating income (loss)
Net realized investment gains (losses), pre-tax
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses)
Net income (loss) from continuing operations