Segment Information (details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment reporting information
Premiums	$ 5,603		$ 5,529		$ 11,120		$ 11,052
Net investment income	687		738		1,357		1,478
Fee income	82		59		179		141
Other revenues	135		29		169		66
Operating income	816		495		1,703		1,296
Net realized investment gains	167	[1]	4	[1]	177	[1]	14	[1]
Total revenues	6,674		6,359		13,002		12,751
Net realized investment gains, net of tax	109		4		118		9
Net income	925		499		1,821		1,305
Reportable Segments [Member]
Segment reporting information
Premiums	5,603		5,529		11,120		11,052
Net investment income	687		738		1,357		1,478
Fee income	82		59		179		141
Other revenues	134		29		170		70
Total operating revenues	6,506		6,355		12,826		12,741
Operating income	875		561		1,825		1,430
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	3,018		2,860		5,960		5,736
Net investment income	502		536		989		1,068
Fee income	82		58		179		140
Other revenues	114		8		127		22
Total operating revenues	3,716		3,462		7,255		6,966
Operating income	579		362		1,169		974
Reportable Segments [Member] | Business Insurance [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	897		754		1,744		1,537
Reportable Segments [Member] | Business Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums	474		488		949		974
Reportable Segments [Member] | Business Insurance [Member] | Commercial Property [Member]
Segment reporting information
Premiums	418		401		827		793
Reportable Segments [Member] | Business Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums	444		437		881		869
Reportable Segments [Member] | Business Insurance [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	776		773		1,541		1,548
Reportable Segments [Member] | Business Insurance [Member] | Other [Member]
Segment reporting information
Premiums	9		7		18		15
Reportable Segments [Member] | Financial, Professional & International Insurance [Member]
Segment reporting information
Premiums	751		766		1,486		1,503
Net investment income	91		99		183		203
Fee income			1				1
Other revenues	5		5		10		13
Total operating revenues	847		871		1,679		1,720
Operating income	154		182		317		331
Reportable Segments [Member] | Financial, Professional & International Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums	221		210		434		417
Reportable Segments [Member] | Financial, Professional & International Insurance [Member] | Other [Member]
Segment reporting information
Premiums	44		42		88		83
Reportable Segments [Member] | Financial, Professional & International Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	228		236		448		459
Reportable Segments [Member] | Financial, Professional & International Insurance [Member] | International [Member]
Segment reporting information
Premiums	258		278		516		544
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	1,834		1,903		3,674		3,813
Net investment income	94		103		185		207
Other revenues	15		16		33		35
Total operating revenues	1,943		2,022		3,892		4,055
Operating income	142		17		339		125
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums	864		918		1,736		1,846
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums	970		985		1,938		1,967
Interest Expense & Other [Member]
Segment reporting information
Other revenues	1				(1)		(4)
Operating income	(59)		(66)		(122)		(134)
After-tax interest expense	$ 56		$ 63		$ 116		$ 125

[1] Total other-than-temporary impairment (OTTI) gains (losses) were $(1) million and $11 million for the three months ended June 30, 2013 and 2012, respectively, and $(1) million and $11 million for the six months ended June 30, 2013 and 2012, respectively. Of total OTTI, credit losses of $(2) million and $(4) million for the three months ended June 30, 2013 and 2012, respectively, and $(7) million and $(8) million for the six months ended June 30, 2013 and 2012, respectively, were recognized in net realized investment gains. In addition, unrealized gains from other changes in total OTTI of $1 million and $15 million for the three months ended June 30, 2013 and 2012, respectively, and $6 million and $19 million for the six months ended June 30, 2013 and 2012, respectively, were recognized in other comprehensive income as part of change in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.