Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating Revenues
Net earned premiums	$ 1,825	$ 1,781	$ 5,389	$ 5,098
Net investment income	597	601	1,808	1,719
Other revenues	78	76	285	230
Total operating revenues	2,500	2,458	7,482	7,047
Claims, Benefits and Expenses
Net incurred claims and benefits	1,408	1,430	4,348	4,150
Policyholders' dividends	6	5	16	14
Amortization of deferred acquisition costs	341	333	1,004	937
Other insurance related expenses	253	268	768	766
Other expenses	121	116	350	338
Total claims, benefits and expenses	2,129	2,152	6,486	6,205
Operating income (loss) before income tax	371	306	996	842
Income tax (expense) benefit on operating income (loss)	(102)	(90)	(292)	(248)
Net operating income (loss)	269	216	704	594
Net realized investment gains (losses), pre-tax	4	8	18	66
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(3)	(6)	(23)
Net realized investment gains (losses)	3	5	12	43
Net income (loss)	272	221	716	637
CNA Specialty [Member]
Operating Revenues
Net earned premiums	768	738	2,237	2,163
Net investment income	159	159	480	446
Other revenues	67	58	190	171
Total operating revenues	994	955	2,907	2,780
Claims, Benefits and Expenses
Net incurred claims and benefits	427	460	1,344	1,376
Policyholders' dividends	3	1	5
Amortization of deferred acquisition costs	161	156	467	458
Other insurance related expenses	64	73	200	222
Other expenses	58	53	172	153
Total claims, benefits and expenses	713	743	2,188	2,209
Operating income (loss) before income tax	281	212	719	571
Income tax (expense) benefit on operating income (loss)	(94)	(76)	(244)	(197)
Net operating income (loss)	187	136	475	374
Net realized investment gains (losses), pre-tax	2	2	(1)	18
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(2)		(6)
Net realized investment gains (losses)	1		(1)	12
Net income (loss)	188	136	474	386
CNA Commercial [Member]
Operating Revenues
Net earned premiums	832	840	2,509	2,452
Net investment income	219	230	680	646
Other revenues	10	11	85	31
Total operating revenues	1,061	1,081	3,274	3,129
Claims, Benefits and Expenses
Net incurred claims and benefits	565	591	1,809	1,749
Policyholders' dividends	1	3	5	9
Amortization of deferred acquisition costs	148	151	450	437
Other insurance related expenses	141	146	426	425
Other expenses	7	7	18	24
Total claims, benefits and expenses	862	898	2,708	2,644
Operating income (loss) before income tax	199	183	566	485
Income tax (expense) benefit on operating income (loss)	(68)	(58)	(198)	(164)
Net operating income (loss)	131	125	368	321
Net realized investment gains (losses), pre-tax	1	10	(7)	34
Income tax (expense) benefit on net realized investment gains (losses)		(3)	3	(12)
Net realized investment gains (losses)	1	7	(4)	22
Net income (loss)	132	132	364	343
Hardy [Member]
Operating Revenues
Net earned premiums	86	64	226	64
Net investment income	1	2	3	2
Other revenues	1		1
Total operating revenues	88	66	230	66
Claims, Benefits and Expenses
Net incurred claims and benefits	35	21	107	21
Policyholders' dividends
Amortization of deferred acquisition costs	25	20	66	20
Other insurance related expenses	14	13	43	13
Other expenses	2	7	15	7
Total claims, benefits and expenses	76	61	231	61
Operating income (loss) before income tax	12	5	(1)	5
Income tax (expense) benefit on operating income (loss)		(2)	3	(2)
Net operating income (loss)	12	3	2	3
Net realized investment gains (losses), pre-tax		(1)	2	(1)
Income tax (expense) benefit on net realized investment gains (losses)	1
Net realized investment gains (losses)	1	(1)	2	(1)
Net income (loss)	13	2	4	2
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	140	141	419	421
Net investment income	209	201	620	600
Other revenues	(3)	1	2	15
Total operating revenues	346	343	1,041	1,036
Claims, Benefits and Expenses
Net incurred claims and benefits	377	350	1,082	1,009
Policyholders' dividends	2	1	6	5
Amortization of deferred acquisition costs	7	6	21	22
Other insurance related expenses	34	35	100	106
Other expenses	5	8	12	18
Total claims, benefits and expenses	425	400	1,221	1,160
Operating income (loss) before income tax	(79)	(57)	(180)	(124)
Income tax (expense) benefit on operating income (loss)	44	35	109	86
Net operating income (loss)	(35)	(22)	(71)	(38)
Net realized investment gains (losses), pre-tax		(3)	14	14
Income tax (expense) benefit on net realized investment gains (losses)		1	(5)	(5)
Net realized investment gains (losses)		(2)	9	9
Net income (loss)	(35)	(24)	(62)	(29)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums		(2)		(1)
Net investment income	9	9	25	25
Other revenues	3	7	8	14
Total operating revenues	12	14	33	38
Claims, Benefits and Expenses
Net incurred claims and benefits	4	8	6	(5)
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	1	1	1	1
Other expenses	49	42	134	137
Total claims, benefits and expenses	54	51	141	133
Operating income (loss) before income tax	(42)	(37)	(108)	(95)
Income tax (expense) benefit on operating income (loss)	16	11	38	29
Net operating income (loss)	(26)	(26)	(70)	(66)
Net realized investment gains (losses), pre-tax	1		10	1
Income tax (expense) benefit on net realized investment gains (losses)	(1)	1	(4)
Net realized investment gains (losses)		1	6	1
Net income (loss)	(26)	(25)	(64)	(65)
Eliminations [Member]
Operating Revenues
Net earned premiums	(1)		(2)	(1)
Net investment income
Other revenues		(1)	(1)	(1)
Total operating revenues	(1)	(1)	(3)	(2)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses	(1)		(2)	(1)
Other expenses		(1)	(1)	(1)
Total claims, benefits and expenses	(1)	(1)	(3)	(2)
Operating income (loss) before income tax
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)