Segment Information (details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment reporting information
Premiums		$ 6,209	$ 6,032	$ 18,257	$ 17,851
Net investment income		582	614	1,675	1,838
Fee income		116	116	352	345
Other revenues		31	21	115	68
Operating income (loss)		701	918	2,048	2,551
Net realized investment gains	[1]	23	15	33	35
Total revenues		6,961	6,798	20,432	20,137
Net realized investment gains, net of tax		15	10	23	22
Net income		716	928	2,071	2,573
Reportable Segments [Member]
Segment reporting information
Premiums		6,209	6,032	18,257	17,851
Net investment income		582	614	1,675	1,838
Fee income		116	116	352	345
Other revenues		28	18	106	65
Total operating revenues		6,935	6,780	20,390	20,099
Operating income (loss)		761	983	2,231	2,742
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums		3,692	3,653	10,922	10,882
Net investment income		445	471	1,280	1,412
Fee income		111	112	340	334
Other revenues		10	5	51	18
Total operating revenues		4,258	4,241	12,593	12,646
Operating income (loss)		457	546	1,326	1,604
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,248	3,184	9,638	9,438
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		1,003	970	2,971	2,889
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		503	485	1,497	1,430
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial Property [Member]
Segment reporting information
Premiums		441	444	1,320	1,324
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General Liability [Member]
Segment reporting information
Premiums		504	484	1,471	1,421
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		788	791	2,356	2,344
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		9	10	23	30
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums		444	469	1,284	1,444
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		529	539	1,555	1,567
Net investment income		53	56	156	169
Other revenues		4	4	13	14
Total operating revenues		586	599	1,724	1,750
Operating income (loss)		146	196	492	471
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums		239	240	708	716
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		245	254	714	719
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums		45	45	133	132
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		1,988	1,840	5,780	5,402
Net investment income		84	87	239	257
Fee income		5	4	12	11
Other revenues		14	9	42	33
Total operating revenues		2,091	1,940	6,073	5,703
Operating income (loss)		158	241	413	667
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums		1,026	893	2,936	2,592
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums		962	947	2,844	2,810
Other [Member]
Segment reporting information
Other revenues		3	3	9	3
Operating income (loss)		(60)	(65)	(183)	(191)
After-tax interest expense		$ 57	$ 61	$ 177	$ 181

[1] Total other-than-temporary impairment (OTTI) losses were $(4) million and $(14) million for the three months ended September 30, 2016 and 2015, respectively, and $(36) million and $(26) million for the nine months ended September 30, 2016 and 2015, respectively. Of total OTTI, credit losses of $(4) million and $(14) million for the three months ended September 30, 2016 and 2015, respectively, and $(26) million and $(23) million for the nine months ended September 30, 2016 and 2015, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $0 million for both of the three months ended September 30, 2016 and 2015, and $(10) million and $(3) million for the nine months ended September 30, 2016 and 2015, 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.