Segment Information (details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment reporting information
Premiums	$ 5,605	$ 5,422	$ 16,479	$ 15,992
Net investment income	690	735	2,227	2,250
Fee income	79	64	227	219
Other revenues	31	35	99	99
Total operating revenues	6,405	6,256	19,032	18,560
Operating income (loss)	332	858	781	2,179
After-tax interest expense	63	62	189	189
Segment Information disclosure
Net realized investment gains	2	226	41	220
Total revenues	6,407	6,482	19,073	18,780
Operating income (loss)	332	858	781	2,179
Net realized investment gains, net of tax	1	147	27	143
Net income	333	1,005	808	2,322
Total reportable segments
Segment reporting information
Premiums	5,605	5,422	16,479	15,992
Net investment income	690	735	2,227	2,250
Fee income	79	64	227	219
Other revenues	31	35	99	99
Total operating revenues	6,405	6,256	19,032	18,560
Operating income (loss)	397	923	995	2,393
Segment Information disclosure
Operating income (loss)	397	923	995	2,393
Business Insurance
Segment reporting information
Premiums	2,890	2,736	8,437	8,027
Net investment income	487	514	1,584	1,579
Fee income	78	64	226	219
Other revenues	8	10	27	23
Total operating revenues	3,463	3,324	10,274	9,848
Operating income (loss)	294	543	909	1,677
Segment Information disclosure
Operating income (loss)	294	543	909	1,677
Workers' compensation
Segment reporting information
Premiums	750	642	2,142	1,847
Commercial automobile
Segment reporting information
Premiums	491	488	1,444	1,430
Property
Segment reporting information
Premiums	405	411	1,207	1,257
Business Insurance, General liability
Segment reporting information
Premiums	442	435	1,299	1,293
Commercial multi-peril
Segment reporting information
Premiums	796	760	2,336	2,200
Business Insurance, Other
Segment reporting information
Premiums	6		9
Financial, Professional & International Insurance
Segment reporting information
Premiums	799	820	2,382	2,499
Net investment income	101	110	312	331
Fee income	1		1
Other revenues	6	7	19	20
Total operating revenues	907	937	2,714	2,850
Operating income (loss)	211	212	495	470
Segment Information disclosure
Operating income (loss)	211	212	495	470
Fidelity and surety
Segment reporting information
Premiums	242	247	726	780
Financial, Professional & International Insurance, General liability
Segment reporting information
Premiums	213	220	637	668
International
Segment reporting information
Premiums	310	319	919	950
Financial, Professional & International Insurance, Other
Segment reporting information
Premiums	34	34	100	101
Personal Insurance
Segment reporting information
Premiums	1,916	1,866	5,660	5,466
Net investment income	102	111	331	340
Other revenues	17	18	53	56
Total operating revenues	2,035	1,995	6,044	5,862
Operating income (loss)	(108)	168	(409)	246
Segment Information disclosure
Operating income (loss)	(108)	168	(409)	246
Automobile
Segment reporting information
Premiums	935	932	2,781	2,757
Homeowners and other
Segment reporting information
Premiums	981	934	2,879	2,709
Interest Expense and Other
Segment reporting information
Operating income (loss)	(65)	(65)	(214)	(214)
Segment Information disclosure
Operating income (loss)	$ (65)	$ (65)	$ (214)	$ (214)

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