Segment Information (details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Segment reporting information
Premiums	$ 5,517		$ 5,523
Net investment income	670		740
Fee income	97		82
Other revenues	34		37
Operating income	887		801
Segment Information disclosure
Net realized investment gains	10	[1]	10	[1]
Total revenues	6,328		6,392
Operating income	887		801
Net realized investment gains, net of tax	9		5
Net income	896		806
Total reportable segments [Member]
Segment reporting information
Premiums	5,517		5,523
Net investment income	670		740
Fee income	97		82
Other revenues	36		41
Total operating revenues	6,320		6,386
Operating income	950		869
Segment Information disclosure
Operating income	950		869
Business Insurance [Member]
Segment reporting information
Premiums	2,942		2,876
Net investment income	487		532
Fee income	97		82
Other revenues	13		14
Total operating revenues	3,539		3,504
Operating income	590		612
Segment Information disclosure
Operating income	590		612
Workers' compensation [Member]
Segment reporting information
Premiums	847		783
Commercial automobile [Member]
Segment reporting information
Premiums	475		486
Commercial property [Member]
Segment reporting information
Premiums	409		392
Business Insurance, General liability [Member]
Segment reporting information
Premiums	437		432
Commercial multi-peril [Member]
Segment reporting information
Premiums	765		775
Business Insurance, Other [Member]
Segment reporting information
Premiums	9		8
Financial, Professional & International Insurance [Member]
Segment reporting information
Premiums	735		737
Net investment income	92		104
Other revenues	5		8
Total operating revenues	832		849
Operating income	163		149
Segment Information disclosure
Operating income	163		149
Fidelity and surety [Member]
Segment reporting information
Premiums	220		223
Financial, Professional & International Insurance, General liability [Member]
Segment reporting information
Premiums	213		207
International [Member]
Segment reporting information
Premiums	258		266
Financial, Professional & International Insurance, Other [Member]
Segment reporting information
Premiums	44		41
Personal Insurance [Member]
Segment reporting information
Premiums	1,840		1,910
Net investment income	91		104
Other revenues	18		19
Total operating revenues	1,949		2,033
Operating income	197		108
Segment Information disclosure
Operating income	197		108
Automobile [Member]
Segment reporting information
Premiums	872		928
Homeowners and other [Member]
Segment reporting information
Premiums	968		982
Interest Expense and Other [Member]
Segment reporting information
Other revenues	(2)		(4)
Operating income	(63)		(68)
After-tax interest expense	60		62
Segment Information disclosure
Operating income	$ (63)		$ (68)

[1] Total other-than-temporary impairment (OTTI) gains were $0 million for each of the three months ended March 31, 2013 and 2012. Of total OTTI, credit losses of $(5) million and $(4) million for the three months ended March 31, 2013 and 2012, respectively, were recognized in net realized investment gains. In addition, unrealized gains from other changes in total OTTI of $5 million and $4 million for the three months ended March 31, 2013 and 2012, respectively, were recognized in other comprehensive income as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.