Segment Information (details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment reporting information
Premiums	$ 5,983		$ 5,666		$ 17,734		$ 16,786
Net investment income	719		657		2,150		2,014
Fee income	110		107		329		286
Other revenues	34		44		109		213
Operating income	893		883		2,618		2,586
Net realized investment gains (losses)	40.0	[1]	(22.0)	[1]	57.0	[1]	155.0	[1]
Total revenues	6,886		6,452		20,379		19,454
Net realized investment gains (losses), net of tax	26		(19)		36		99
Net income	919		864		2,654		2,685
Reportable Segments [Member]
Segment reporting information
Premiums	5,983		5,666		17,734		16,786
Net investment income	719		657		2,150		2,014
Fee income	110		107		329		286
Other revenues	34		47		109		217
Total operating revenues	6,846		6,477		20,322		19,303
Operating income	956		948		2,810		2,773
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums	3,660		3,325		10,849		9,801
Net investment income	557		504		1,666		1,544
Fee income	110		107		329		286
Other revenues	10		8		32		135
Total operating revenues	4,337		3,944		12,876		11,766
Operating income	552		566		1,717		1,773
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums	553		279		1,659		795
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,107		3,046		9,190		9,006
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	928		896		2,759		2,640
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	481		476		1,422		1,425
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial Property [Member]
Segment reporting information
Premiums	442		432		1,310		1,259
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General Liability [Member]
Segment reporting information
Premiums	472		452		1,376		1,333
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	774		779		2,292		2,320
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	10		11		31		29
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	527		506		1,554		1,476
Net investment income	64		63		192		195
Other revenues	5		5		15		15
Total operating revenues	596		574		1,761		1,686
Operating income	165		120		511		399
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums	243		227		721		661
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums	44		44		133		132
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	240		235		700		683
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	1,796		1,835		5,331		5,509
Net investment income	98		90		292		275
Other revenues	19		34		62		67
Total operating revenues	1,913		1,959		5,685		5,851
Operating income	239		262		582		601
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums	835		855		2,471		2,591
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums	961		980		2,860		2,918
Interest Expense & Other [Member]
Segment reporting information
Other revenues			(3)				(4)
Operating income	(63)		(65)		(192)		(187)
After-tax interest expense	$ 60		$ 59		$ 180		$ 175

[1] Total other-than-temporary impairment (OTTI) gains (losses) were $(8) million and $0 million for the three months ended September 30, 2014 and 2013, respectively, and $(16) million and $(1) million for the nine months ended September 30, 2014 and 2013, respectively. Of total OTTI, credit losses of $(10) million and $(3) million for the three months ended September 30, 2014 and 2013, respectively, and $(20) million and $(10) million for the nine months ended September 30, 2014 and 2013, respectively, were recognized in net realized investment gains (losses). In addition, unrealized gains from other changes in total OTTI of $2 million and $3 million for the three months ended September 30, 2014 and 2013, respectively, and $4 million and $9 million for the nine months ended September 30, 2014 and 2013, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.