Segment Information (details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment reporting information
Premiums		$ 5,931	$ 5,928	$ 11,819	$ 11,751
Net investment income		632	695	1,224	1,431
Fee income		111	112	222	219
Other revenues		22	34	47	75
Operating income (loss)		806	673	1,633	1,725
Net realized investment gains	[1]	10	16	20	17
Total revenues		6,706	6,785	13,332	13,493
Net realized investment gains, net of tax		6	10	12	10
Net income		812	683	1,645	1,735
Reportable Segments [Member]
Segment reporting information
Premiums		5,931	5,928	11,819	11,751
Net investment income		632	695	1,224	1,431
Fee income		111	112	222	219
Other revenues		22	33	47	75
Total operating revenues		6,696	6,768	13,312	13,476
Operating income (loss)		868	738	1,759	1,854
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums		3,609	3,631	7,229	7,189
Net investment income		487	539	941	1,109
Fee income		111	112	222	219
Other revenues		5	10	13	22
Total operating revenues		4,212	4,292	8,405	8,539
Operating income (loss)		543	471	1,058	1,165
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,138	3,067	6,267	6,083
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		957	923	1,919	1,831
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		477	473	946	941
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial Property [Member]
Segment reporting information
Premiums		442	440	883	868
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General Liability [Member]
Segment reporting information
Premiums		473	457	945	904
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		779	763	1,554	1,518
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		10	11	20	21
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums		471	564	962	1,106
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		524	524	1,028	1,027
Net investment income		57	62	113	128
Other revenues		5	6	10	10
Total operating revenues		586	592	1,151	1,165
Operating income (loss)		151	192	275	346
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums		240	241	476	478
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		240	238	465	460
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums		44	45	87	89
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		1,798	1,773	3,562	3,535
Net investment income		88	94	170	194
Other revenues		12	17	24	43
Total operating revenues		1,898	1,884	3,756	3,772
Operating income (loss)		174	75	426	343
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums		863	821	1,699	1,636
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums		935	952	1,863	1,899
Interest Expense & Other [Member]
Segment reporting information
Other revenues			1
Operating income (loss)		(62)	(65)	(126)	(129)
After-tax interest expense		$ 60	$ 60	$ 120	$ 120

[1] Total other-than-temporary impairment (OTTI) losses were $(8) million and $(1) million for the three months ended June 30, 2015 and 2014, respectively, and $(12) million and $(8) million for the six months ended June 30, 2015 and 2014, respectively. Of total OTTI, credit losses of $(6) million and $(1) million for the three months ended June 30, 2015 and 2014, respectively, and $(9) million and $(10) million for the six months ended June 30, 2015 and 2014, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $(2) million and $0 million for the three months ended June 30, 2015 and 2014, respectively, and $(3) million and $2 million for the six months ended June 30, 2015 and 2014, 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.