Segment Information (details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment reporting information
Premiums	$ 5,529	$ 5,503	$ 11,052	$ 10,874
Net investment income	738	758	1,478	1,537
Fee income	59	74	141	148
Other revenues	29	34	66	68
Operating income (loss)	495	(377)	1,296	449
Segment Information disclosure
Net realized investment gains	4	19	14	39
Total consolidated revenues	6,359	6,388	12,751	12,666
Operating income (loss)	495	(377)	1,296	449
Net realized investment gains, net of tax	4	13	9	26
Net income (loss)	499	(364)	1,305	475
Total reportable segments [Member]
Segment reporting information
Premiums	5,529	5,503	11,052	10,874
Net investment income	738	758	1,478	1,537
Fee income	59	74	141	148
Other revenues	29	34	70	68
Total operating revenues	6,355	6,369	12,741	12,627
Operating income (loss)	561	(296)	1,430	598
Segment Information disclosure
Operating income (loss)	561	(296)	1,430	598
Business Insurance [Member]
Segment reporting information
Premiums	2,860	2,802	5,736	5,547
Net investment income	536	541	1,068	1,097
Fee income	58	74	140	148
Other revenues	8	10	22	19
Total operating revenues	3,462	3,427	6,966	6,811
Operating income (loss)	362	11	974	615
Segment Information disclosure
Operating income (loss)	362	11	974	615
Workers' compensation [Member]
Segment reporting information
Premiums	754	712	1,537	1,392
Commercial automobile [Member]
Segment reporting information
Premiums	488	480	974	953
Property [Member]
Segment reporting information
Premiums	401	401	793	802
Business Insurance, General liability [Member]
Segment reporting information
Premiums	437	429	869	857
Commercial multi-peril [Member]
Segment reporting information
Premiums	773	778	1,548	1,540
Business Insurance, Other [Member]
Segment reporting information
Premiums	7	2	15	3
Financial, Professional & International Insurance [Member]
Segment reporting information
Premiums	766	810	1,503	1,583
Net investment income	99	105	203	211
Fee income	1		1
Other revenues	5	6	13	13
Total operating revenues	871	921	1,720	1,807
Operating income (loss)	182	164	331	284
Segment Information disclosure
Operating income (loss)	182	164	331	284
Fidelity and surety [Member]
Segment reporting information
Premiums	236	249	459	483
Financial, Professional & International Insurance, General liability [Member]
Segment reporting information
Premiums	210	211	417	415
International [Member]
Segment reporting information
Premiums	278	311	544	609
Financial, Professional & International Insurance, Other [Member]
Segment reporting information
Premiums	42	39	83	76
Personal Insurance [Member]
Segment reporting information
Premiums	1,903	1,891	3,813	3,744
Net investment income	103	112	207	229
Other revenues	16	18	35	36
Total operating revenues	2,022	2,021	4,055	4,009
Operating income (loss)	17	(471)	125	(301)
Segment Information disclosure
Operating income (loss)	17	(471)	125	(301)
Automobile [Member]
Segment reporting information
Premiums	918	931	1,846	1,846
Homeowners and other [Member]
Segment reporting information
Premiums	985	960	1,967	1,898
Interest Expense and Other [Member]
Segment reporting information
Other revenues			(4)
Operating income (loss)	(66)	(81)	(134)	(149)
After-tax interest expense	63	63	125	125
Segment Information disclosure
Operating income (loss)	$ (66)	$ (81)	$ (134)	$ (149)