Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment reporting information
Premiums		$ 6,695	$ 6,351	$ 13,232	$ 12,534
Net investment income		595	598	1,198	1,208
Fee income		112	116	215	229
Other revenues		39	39	93	70
Net realized investment gains	[1]	36	80	25	85
Total revenues		7,477	7,184	14,763	14,126
Core income (loss)		494	543	1,172	1,157
Net realized investment gains, net of tax		30	52	21	55
Net income		524	595	1,193	1,212
Reportable Segments [Member]
Segment reporting information
Premiums		6,695	6,351	13,232	12,534
Net investment income		595	598	1,198	1,208
Fee income		112	116	215	229
Other revenues		39	36	93	66
Total segment revenues		7,441	7,101	14,738	14,037
Core income (loss)		572	604	1,326	1,280
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums		3,641	3,504	7,209	6,933
Net investment income		440	447	886	900
Fee income		107	112	206	221
Other revenues		20	15	51	24
Total segment revenues		4,208	4,078	8,352	8,078
Core income (loss)		385	429	837	871
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,376	3,265	6,680	6,454
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		973	999	1,944	1,975
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		587	521	1,149	1,027
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums		453	443	891	878
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		535	498	1,056	989
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		822	797	1,627	1,571
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		6	7	13	14
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums		265	239	529	479
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		601	575	1,183	1,130
Net investment income		57	56	115	117
Other revenues		5	6	11	11
Total segment revenues		663	637	1,309	1,258
Core income (loss)		204	163	377	308
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		550	530	1,085	1,044
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		248	239	490	474
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		253	245	499	479
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		49	46	96	91
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums		51	45	98	86
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		2,453	2,272	4,840	4,471
Net investment income		98	95	197	191
Fee income		5	4	9	8
Other revenues		14	15	31	31
Total segment revenues		2,570	2,386	5,077	4,701
Core income (loss)		(17)	12	112	101
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		2,283	2,122	4,503	4,171
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		1,261	1,145	2,486	2,239
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums		1,022	977	2,017	1,932
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums		170	150	337	300
Other [Member]
Segment reporting information
Other revenues			3		4
Core income (loss)		(78)	(61)	(154)	(123)
After-tax interest expense		$ 71	$ 60	$ 141	$ 118

[1] Total other-than-temporary impairment (OTTI) losses were $(1) million and $(5) million for the three months ended June 30, 2018 and 2017, respectively, and $(1) million and $(6) million for the six months ended June 30, 2018 and 2017, respectively. Of total OTTI, credit losses of $(1) million and $(5) million for the three months ended June 30, 2018 and 2017, respectively, and $(1) million and $(7) million for the six months ended June 30, 2018 and 2017, 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, 2018 and 2017, respectively, and $0 million and $1 million for the six months ended June 30, 2018 and 2017, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income.