Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating Revenues
Net earned premiums	$ 1,764	$ 1,649
Net investment income	633	648
Other revenues	78	68
Total operating revenues	2,475	2,365
Claims, Benefits and Expenses
Net incurred claims and benefits	1,425	1,378
Policyholders' dividends	4	3
Amortization of deferred acquisition costs	328	295
Other insurance related expenses	262	250
Other expenses	121	111
Total claims, benefits and expenses	2,140	2,037
Operating income (loss) before income tax	335	328
Income tax (expense) benefit on operating income (loss)	(104)	(102)
Net operating income (loss)	231	226
Net realized investment gains (losses)	28	36
Income tax (expense) benefit on net realized investment gains (losses)	(9)	(12)
Net realized investment gains (losses), after-tax	19	24
Net income (loss)	250	250
CNA Specialty [Member]
Operating Revenues
Net earned premiums	723	706
Net investment income	170	175
Other revenues	60	56
Total operating revenues	953	937
Claims, Benefits and Expenses
Net incurred claims and benefits	465	468
Policyholders' dividends	1	(2)
Amortization of deferred acquisition costs	150	148
Other insurance related expenses	71	72
Other expenses	54	50
Total claims, benefits and expenses	741	736
Operating income (loss) before income tax	212	201
Income tax (expense) benefit on operating income (loss)	(72)	(69)
Net operating income (loss)	140	132
Net realized investment gains (losses)	3	8
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(2)
Net realized investment gains (losses), after-tax	2	6
Net income (loss)	142	138
CNA Commercial [Member]
Operating Revenues
Net earned premiums	837	803
Net investment income	250	265
Other revenues	10	9
Total operating revenues	1,097	1,077
Claims, Benefits and Expenses
Net incurred claims and benefits	600	567
Policyholders' dividends	2	3
Amortization of deferred acquisition costs	149	139
Other insurance related expenses	143	144
Other expenses	9	7
Total claims, benefits and expenses	903	860
Operating income (loss) before income tax	194	217
Income tax (expense) benefit on operating income (loss)	(69)	(78)
Net operating income (loss)	125	139
Net realized investment gains (losses)	4	11
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(4)
Net realized investment gains (losses), after-tax	3	7
Net income (loss)	128	146
Hardy [Member]
Operating Revenues
Net earned premiums	64
Net investment income	1
Other revenues	1
Total operating revenues	66
Claims, Benefits and Expenses
Net incurred claims and benefits	31
Policyholders' dividends
Amortization of deferred acquisition costs	21
Other insurance related expenses	16
Other expenses	7
Total claims, benefits and expenses	75
Operating income (loss) before income tax	(9)
Income tax (expense) benefit on operating income (loss)	1
Net operating income (loss)	(8)
Net realized investment gains (losses)	1
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses), after-tax	1
Net income (loss)	(7)
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	141	141
Net investment income	204	198
Other revenues	4	(2)
Total operating revenues	349	337
Claims, Benefits and Expenses
Net incurred claims and benefits	325	336
Policyholders' dividends	1	2
Amortization of deferred acquisition costs	8	8
Other insurance related expenses	33	35
Other expenses	5	6
Total claims, benefits and expenses	372	387
Operating income (loss) before income tax	(23)	(50)
Income tax (expense) benefit on operating income (loss)	23	31
Net operating income (loss)		(19)
Net realized investment gains (losses)	14	13
Income tax (expense) benefit on net realized investment gains (losses)	(5)	(5)
Net realized investment gains (losses), after-tax	9	8
Net income (loss)	9	(11)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums		(1)
Net investment income	8	10
Other revenues	3	5
Total operating revenues	11	14
Claims, Benefits and Expenses
Net incurred claims and benefits	4	7
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses		(1)
Other expenses	46	48
Total claims, benefits and expenses	50	54
Operating income (loss) before income tax	(39)	(40)
Income tax (expense) benefit on operating income (loss)	13	14
Net operating income (loss)	(26)	(26)
Net realized investment gains (losses)	6	4
Income tax (expense) benefit on net realized investment gains (losses)	(2)	(1)
Net realized investment gains (losses), after-tax	4	3
Net income (loss)	(22)	(23)
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) before income tax
Income tax (expense) benefit on operating income (loss)
Net operating income (loss)
Net realized investment gains (losses)
Income tax (expense) benefit on net realized investment gains (losses)
Net realized investment gains (losses), after-tax
Net income (loss)