Segment Information (details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment reporting information
Premiums	$ 5,523	$ 5,371
Net investment income	740	779
Fee income	82	74
Other revenues	37	34
Operating income	801	826
Segment Information disclosure
Net realized investment gains	10	20
Total consolidated revenues	6,392	6,278
Operating income	801	826
Net realized investment gains, net of tax	5	13
Net income	806	839
Total reportable segments [Member]
Segment reporting information
Premiums	5,523	5,371
Net investment income	740	779
Fee income	82	74
Other revenues	41	34
Total operating revenues	6,386	6,258
Operating income	869	894
Segment Information disclosure
Operating income	869	894
Business Insurance [Member]
Segment reporting information
Premiums	2,876	2,745
Net investment income	532	556
Fee income	82	74
Other revenues	14	9
Total operating revenues	3,504	3,384
Operating income	612	604
Segment Information disclosure
Operating income	612	604
Workers' compensation [Member]
Segment reporting information
Premiums	783	680
Commercial automobile [Member]
Segment reporting information
Premiums	486	473
Property [Member]
Segment reporting information
Premiums	392	401
Business Insurance, General liability [Member]
Segment reporting information
Premiums	432	428
Commercial multi-peril [Member]
Segment reporting information
Premiums	775	762
Business Insurance, Other [Member]
Segment reporting information
Premiums	8	1
Financial, Professional & International Insurance [Member]
Segment reporting information
Premiums	737	773
Net investment income	104	106
Other revenues	8	7
Total operating revenues	849	886
Operating income	149	120
Segment Information disclosure
Operating income	149	120
Fidelity and surety [Member]
Segment reporting information
Premiums	223	234
Financial, Professional & International Insurance, General liability [Member]
Segment reporting information
Premiums	207	204
International [Member]
Segment reporting information
Premiums	266	298
Financial, Professional & International Insurance, Other [Member]
Segment reporting information
Premiums	41	37
Personal Insurance [Member]
Segment reporting information
Premiums	1,910	1,853
Net investment income	104	117
Other revenues	19	18
Total operating revenues	2,033	1,988
Operating income	108	170
Segment Information disclosure
Operating income	108	170
Automobile [Member]
Segment reporting information
Premiums	928	915
Homeowners and other [Member]
Segment reporting information
Premiums	982	938
Interest Expense and Other [Member]
Segment reporting information
Other revenues	(4)
Operating income	(68)	(68)
After-tax interest expense	62	62
Segment Information disclosure
Operating income	$ (68)	$ (68)