Segment Information (details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment reporting information
Premiums	$ 5,666	$ 5,605	$ 16,718	$ 16,479
Net investment income	722	690	2,200	2,227
Fee income	92	79	233	227
Other revenues	34	31	100	99
Operating income (loss)	867	332	2,163	781
Segment Information disclosure
Net realized investment gains	(2)	2	12	41
Total consolidated revenues	6,512	6,407	19,263	19,073
Operating income	867	332	2,163	781
Net realized investment gains, net of tax	(3)	1	6	27
Net income	864	333	2,169	808
Total reportable segments [Member]
Segment reporting information
Premiums	5,666	5,605	16,718	16,479
Net investment income	722	690	2,200	2,227
Fee income	92	79	233	227
Other revenues	34	31	104	99
Total operating revenues	6,514	6,405	19,255	19,032
Operating income (loss)	929	397	2,359	995
Segment Information disclosure
Operating income	929	397	2,359	995
Business Insurance [Member]
Segment reporting information
Premiums	2,982	2,890	8,718	8,437
Net investment income	524	487	1,592	1,584
Fee income	92	78	232	226
Other revenues	9	8	31	27
Total operating revenues	3,607	3,463	10,573	10,274
Operating income (loss)	543	294	1,517	909
Segment Information disclosure
Operating income	543	294	1,517	909
Workers' compensation [Member]
Segment reporting information
Premiums	841	750	2,378	2,142
Commercial automobile [Member]
Segment reporting information
Premiums	488	491	1,462	1,444
Property [Member]
Segment reporting information
Premiums	414	405	1,207	1,207
Business Insurance, General liability [Member]
Segment reporting information
Premiums	444	442	1,313	1,299
Commercial multi-peril [Member]
Segment reporting information
Premiums	785	796	2,333	2,336
Business Insurance, Other [Member]
Segment reporting information
Premiums	10	6	25	9
Financial, Professional & International Insurance [Member]
Segment reporting information
Premiums	772	799	2,275	2,382
Net investment income	97	101	300	312
Fee income		1	1	1
Other revenues	8	6	21	19
Total operating revenues	877	907	2,597	2,714
Operating income (loss)	180	211	511	495
Segment Information disclosure
Operating income	180	211	511	495
Fidelity and surety [Member]
Segment reporting information
Premiums	240	241	699	724
Financial, Professional & International Insurance, General liability [Member]
Segment reporting information
Premiums	216	208	633	623
International [Member]
Segment reporting information
Premiums	274	311	818	920
Financial, Professional & International Insurance, Other [Member]
Segment reporting information
Premiums	42	39	125	115
Personal Insurance [Member]
Segment reporting information
Premiums	1,912	1,916	5,725	5,660
Net investment income	101	102	308	331
Other revenues	17	17	52	53
Total operating revenues	2,030	2,035	6,085	6,044
Operating income (loss)	206	(108)	331	(409)
Segment Information disclosure
Operating income	206	(108)	331	(409)
Automobile [Member]
Segment reporting information
Premiums	916	935	2,762	2,781
Homeowners and other [Member]
Segment reporting information
Premiums	996	981	2,963	2,879
Interest Expense and Other [Member]
Segment reporting information
Other revenues			(4)
Operating income (loss)	(62)	(65)	(196)	(214)
After-tax interest expense	60	63	185	189
Segment Information disclosure
Operating income	$ (62)	$ (65)	$ (196)	$ (214)