Business Segments (Income Statement Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating Revenues
Net earned premiums	$ 1,811	$ 1,800	$ 3,617	$ 3,564
Net investment income	550	534	1,076	1,125
Other revenues	93	130	178	207
Total operating revenues	2,454	2,464	4,871	4,896
Claims, Benefits and Expenses
Net incurred claims and benefits	1,437	1,482	2,880	2,875
Policyholders' dividends	4	3	7	6
Amortization of deferred acquisition costs	335	335	664	663
Other insurance related expenses	259	252	521	513
Other expenses	41	108	169	229
Total claims, benefits and expenses	2,076	2,180	4,241	4,286
Operating income (loss) before income tax	378	284	630	610
Income tax (expense) benefit on operating income (loss)	(106)	(85)	(168)	(186)
Net operating income (loss)	272	199	462	424
Net realized investment gains (losses), pre-tax	(14)	(14)	32	9
Income tax (expense) benefit on net realized investment gains (losses)	3	5	(13)	(2)
Net realized investment gains (losses)	(11)	(9)	19	7
Net income (loss) from continuing operations	261	190	481	431
CNA Specialty [Member]
Operating Revenues
Net earned premiums	763	746	1,511	1,469
Net investment income	161	151	312	321
Other revenues	74	63	142	123
Total operating revenues	998	960	1,965	1,913
Claims, Benefits and Expenses
Net incurred claims and benefits	426	452	906	917
Policyholders' dividends	1	1	2	2
Amortization of deferred acquisition costs	158	156	311	306
Other insurance related expenses	74	65	146	136
Other expenses	65	60	126	114
Total claims, benefits and expenses	724	734	1,491	1,475
Operating income (loss) before income tax	274	226	474	438
Income tax (expense) benefit on operating income (loss)	(92)	(78)	(158)	(150)
Net operating income (loss)	182	148	316	288
Net realized investment gains (losses), pre-tax	(5)	(6)	7	(3)
Income tax (expense) benefit on net realized investment gains (losses)	2	2	(2)	1
Net realized investment gains (losses)	(3)	(4)	5	(2)
Net income (loss) from continuing operations	179	144	321	286
CNA Commercial [Member]
Operating Revenues
Net earned premiums	817	840	1,639	1,677
Net investment income	208	211	406	461
Other revenues	9	65	18	75
Total operating revenues	1,034	1,116	2,063	2,213
Claims, Benefits and Expenses
Net incurred claims and benefits	654	644	1,276	1,244
Policyholders' dividends	3	2	5	4
Amortization of deferred acquisition costs	144	153	284	302
Other insurance related expenses	138	142	277	285
Other expenses	8	2	19	11
Total claims, benefits and expenses	947	943	1,861	1,846
Operating income (loss) before income tax	87	173	202	367
Income tax (expense) benefit on operating income (loss)	(28)	(61)	(65)	(130)
Net operating income (loss)	59	112	137	237
Net realized investment gains (losses), pre-tax	(7)	(12)	5	(8)
Income tax (expense) benefit on net realized investment gains (losses)	1	4	(3)	3
Net realized investment gains (losses)	(6)	(8)	2	(5)
Net income (loss) from continuing operations	53	104	139	232
Hardy [Member]
Operating Revenues
Net earned premiums	92	76	190	140
Net investment income	1	1	2	2
Other revenues	(1)	(1)
Total operating revenues	92	76	192	142
Claims, Benefits and Expenses
Net incurred claims and benefits	52	41	90	72
Policyholders' dividends
Amortization of deferred acquisition costs	26	20	54	41
Other insurance related expenses	16	13	36	29
Other expenses	4	6	8	13
Total claims, benefits and expenses	98	80	188	155
Operating income (loss) before income tax	(6)	(4)	4	(13)
Income tax (expense) benefit on operating income (loss)	1	2	(2)	3
Net operating income (loss)	(5)	(2)	2	(10)
Net realized investment gains (losses), pre-tax	(1)	1	(1)	2
Income tax (expense) benefit on net realized investment gains (losses)		(1)		(1)
Net realized investment gains (losses)	(1)		(1)	1
Net income (loss) from continuing operations	(6)	(2)	1	(9)
Life and Group Non-Core [Member]
Operating Revenues
Net earned premiums	139	138	278	279
Net investment income	173	163	344	325
Other revenues	8	2	13	5
Total operating revenues	320	303	635	609
Claims, Benefits and Expenses
Net incurred claims and benefits	304	347	610	640
Policyholders' dividends
Amortization of deferred acquisition costs	7	6	15	14
Other insurance related expenses	31	32	63	64
Other expenses	(3)	2	(2)	7
Total claims, benefits and expenses	339	387	686	725
Operating income (loss) before income tax	(19)	(84)	(51)	(116)
Income tax (expense) benefit on operating income (loss)	28	43	58	69
Net operating income (loss)	9	(41)	7	(47)
Net realized investment gains (losses), pre-tax	(4)		12	9
Income tax (expense) benefit on net realized investment gains (losses)	1	1	(5)	(2)
Net realized investment gains (losses)	(3)	1	7	7
Net income (loss) from continuing operations	6	(40)	14	(40)
Corporate and Other Non-Core [Member]
Operating Revenues
Net earned premiums
Net investment income	7	8	12	16
Other revenues	5	2	7	5
Total operating revenues	12	10	19	21
Claims, Benefits and Expenses
Net incurred claims and benefits	1	(2)	(2)	2
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses
Other expenses	(31)	39	20	85
Total claims, benefits and expenses	(30)	37	18	87
Operating income (loss) before income tax	42	(27)	1	(66)
Income tax (expense) benefit on operating income (loss)	(15)	9	(1)	22
Net operating income (loss)	27	(18)		(44)
Net realized investment gains (losses), pre-tax	3	3	9	9
Income tax (expense) benefit on net realized investment gains (losses)	(1)	(1)	(3)	(3)
Net realized investment gains (losses)	2	2	6	6
Net income (loss) from continuing operations	29	(16)	6	(38)
Eliminations [Member]
Operating Revenues
Net earned premiums			(1)	(1)
Net investment income
Other revenues	(2)	(1)	(2)	(1)
Total operating revenues	(2)	(1)	(3)	(2)
Claims, Benefits and Expenses
Net incurred claims and benefits
Policyholders' dividends
Amortization of deferred acquisition costs
Other insurance related expenses			(1)	(1)
Other expenses	(2)	(1)	(2)	(1)
Total claims, benefits and expenses	(2)	(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) from continuing operations