Segment Information (details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment reporting information
Premiums		$ 6,067	$ 5,931	$ 12,048	$ 11,819
Net investment income		549	632	1,093	1,224
Fee income		119	115	236	229
Other revenues		31	22	84	47
Operating income (loss)		649	806	1,347	1,633
Net realized investment gains	[1]	19	10	10	20
Total revenues		6,785	6,710	13,471	13,339
Net realized investment gains, net of tax		15	6	8	12
Net income		664	812	1,355	1,645
Reportable Segments [Member]
Segment reporting information
Premiums		6,067	5,931	12,048	11,819
Net investment income		549	632	1,093	1,224
Fee income		119	115	236	229
Other revenues		28	22	78	47
Total operating revenues		6,763	6,700	13,455	13,319
Operating income (loss)		711	868	1,470	1,759
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums		3,631	3,609	7,230	7,229
Net investment income		420	487	835	941
Fee income		115	111	229	222
Other revenues		8	5	41	13
Total operating revenues		4,174	4,212	8,335	8,405
Operating income (loss)		393	543	869	1,058
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,212	3,132	6,390	6,254
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		987	957	1,968	1,919
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		503	477	994	945
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial Property [Member]
Segment reporting information
Premiums		442	440	879	880
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General Liability [Member]
Segment reporting information
Premiums		485	469	967	937
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		786	779	1,568	1,553
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		9	10	14	20
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums		419	477	840	975
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		518	524	1,026	1,028
Net investment income		51	57	103	113
Other revenues		6	5	9	10
Total operating revenues		575	586	1,138	1,151
Operating income (loss)		202	151	346	275
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums		235	240	469	476
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		239	240	469	465
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums		44	44	88	87
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		1,918	1,798	3,792	3,562
Net investment income		78	88	155	170
Fee income		4	4	7	7
Other revenues		14	12	28	24
Total operating revenues		2,014	1,902	3,982	3,763
Operating income (loss)		116	174	255	426
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums		974	863	1,910	1,699
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums		944	935	1,882	1,863
Other [Member]
Segment reporting information
Other revenues		3		6
Operating income (loss)		(62)	(62)	(123)	(126)
After-tax interest expense		$ 60	$ 60	$ 120	$ 120

[1] Total other-than-temporary impairment (OTTI) losses were $(4) million and $(8) million for the three months ended June 30, 2016 and 2015, respectively, and $(32) million and $(12) million for the six months ended June 30, 2016 and 2015, respectively. Of total OTTI, credit losses of $(4) million and $(6) million for the three months ended June 30, 2016 and 2015, respectively, and $(22) million and $(9) million for the six months ended June 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 and $(2) million for the three months ended June 30, 2016 and 2015, respectively, and $(10) million and $(3) million for the six months ended June 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.