Segment Information (details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment reporting information
Premiums		$ 6,032	$ 5,983	$ 17,851	$ 17,734
Net investment income		614	719	1,838	2,150
Fee income		112	110	334	329
Other revenues		21	34	68	109
Operating income (loss)		918	893	2,551	2,618
Net realized investment gains	[1]	15	40	35	57
Total revenues		6,794	6,886	20,126	20,379
Net realized investment gains, net of tax		10	26	22	36
Net income		928	919	2,573	2,654
Reportable Segments [Member]
Segment reporting information
Premiums		6,032	5,983	17,851	17,734
Net investment income		614	719	1,838	2,150
Fee income		112	110	334	329
Other revenues		18	34	65	109
Total operating revenues		6,776	6,846	20,088	20,322
Operating income (loss)		983	956	2,742	2,810
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums		3,653	3,660	10,882	10,849
Net investment income		471	557	1,412	1,666
Fee income		112	110	334	329
Other revenues		5	10	18	32
Total operating revenues		4,241	4,337	12,646	12,876
Operating income (loss)		546	552	1,604	1,717
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,189	3,107	9,456	9,190
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		970	928	2,889	2,759
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		486	481	1,432	1,422
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums		446	442	1,329	1,310
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		487	472	1,432	1,376
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		790	774	2,344	2,292
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		10	10	30	31
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums		464	553	1,426	1,659
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		539	527	1,567	1,554
Net investment income		56	64	169	192
Other revenues		4	5	14	15
Total operating revenues		599	596	1,750	1,761
Operating income (loss)		196	165	471	511
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General liability [Member]
Segment reporting information
Premiums		240	243	716	721
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		254	240	719	700
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums		45	44	132	133
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		1,840	1,796	5,402	5,331
Net investment income		87	98	257	292
Other revenues		9	19	33	62
Total operating revenues		1,936	1,913	5,692	5,685
Operating income (loss)		241	239	667	582
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums		893	835	2,592	2,471
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums		947	961	2,810	2,860
Interest Expense & Other [Member]
Segment reporting information
Other revenues		3		3
Operating income (loss)		(65)	(63)	(191)	(192)
After-tax interest expense		$ 61	$ 60	$ 181	$ 180

[1] Total other-than-temporary impairment (OTTI) losses were $(14) million and $(8) million for the three months ended September 30, 2015 and 2014, respectively, and $(26) million and $(16) million for the nine months ended September 30, 2015 and 2014, respectively. Of total OTTI, credit losses of $(14) million and $(10) million for the three months ended September 30, 2015 and 2014, respectively, and $(23) million and $(20) million for the nine months ended September 30, 2015 and 2014, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $0 million and $2 million for the three months ended September 30, 2015 and 2014, respectively, and $(3) million and $4 million for the nine months ended September 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.