Segment Information (details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment reporting information
Premiums		$ 6,351	$ 6,067	$ 12,534	$ 12,048
Net investment income		598	549	1,208	1,093
Fee income		116	119	229	236
Other revenues		39	31	70	84
Core income (loss)		543	649	1,157	1,347
Net realized investment gains	[1]	80	19	85	10
Total revenues		7,184	6,785	14,126	13,471
Net realized investment gains, net of tax		52	15	55	8
Net income		595	664	1,212	1,355
Reportable Segments [Member]
Segment reporting information
Premiums		6,351	6,067	12,534	12,048
Net investment income		598	549	1,208	1,093
Fee income		116	119	229	236
Other revenues		36	28	66	78
Total segment revenues		7,101	6,763	14,037	13,455
Core income (loss)		604	711	1,280	1,470
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums		3,504	3,439	6,933	6,853
Net investment income		447	404	900	803
Fee income		112	115	221	229
Other revenues		15	8	24	38
Total segment revenues		4,078	3,966	8,078	7,923
Core income (loss)		429	401	871	848
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,265	3,212	6,454	6,390
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		999	987	1,975	1,968
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		521	503	1,027	994
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums		443	442	878	879
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		498	485	989	967
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		797	786	1,571	1,568
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		7	9	14	14
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums		239	227	479	463
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		575	559	1,130	1,111
Net investment income		56	58	117	118
Other revenues		6	5	11	9
Total segment revenues		637	622	1,258	1,238
Core income (loss)		163	215	308	375
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		530	518	1,044	1,026
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		239	235	474	469
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		245	239	479	469
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		46	44	91	88
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums		45	41	86	85
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		2,272	2,069	4,471	4,084
Net investment income		95	87	191	172
Fee income		4	4	8	7
Other revenues		15	15	31	31
Total segment revenues		2,386	2,175	4,701	4,294
Core income (loss)		12	95	101	247
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		2,122	1,918	4,171	3,792
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		1,145	974	2,239	1,910
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums		977	944	1,932	1,882
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums		150	151	300	292
Other [Member]
Segment reporting information
Other revenues		3	3	4	6
Core income (loss)		(61)	(62)	(123)	(123)
After-tax interest expense		$ 60	$ 60	$ 118	$ 120

[1] Total other-than-temporary impairment (OTTI) losses were $(5) million and $(4) million for the three months ended June 30, 2017 and 2016, respectively, and $(6) million and $(32) million for the six months ended June 30, 2017 and 2016, respectively. Of total OTTI, credit losses of $(5) million and $(4) million for the three months ended June 30, 2017 and 2016, respectively, and $(7) million and $(22) million for the six months ended June 30, 2017 and 2016, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $0 million for each of the three months ended June 30, 2017 and 2016, and $1 million and $(10) million for the six months ended June 30, 2017 and 2016, respectively, were recognized in other comprehensive income as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.