Segment Information (details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment reporting information
Premiums		$ 6,523	$ 6,209	$ 19,057	$ 18,257
Net investment income		588	582	1,796	1,675
Fee income		113	116	342	352
Other revenues		40	31	110	115
Core income (loss)		253	701	1,410	2,048
Net realized investment gains	[1]	61	23	146	33
Total revenues		7,325	6,961	21,451	20,432
Net realized investment gains, net of tax		40	15	95	23
Net income		293	716	1,505	2,071
Reportable Segments [Member]
Segment reporting information
Premiums		6,523	6,209	19,057	18,257
Net investment income		588	582	1,796	1,675
Fee income		113	116	342	352
Other revenues		38	28	104	106
Total segment revenues		7,262	6,935	21,299	20,390
Core income (loss)		318	761	1,598	2,231
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums		3,576	3,497	10,509	10,350
Net investment income		437	431	1,337	1,234
Fee income		108	111	329	340
Other revenues		19	7	43	45
Total segment revenues		4,140	4,046	12,218	11,969
Core income (loss)		105	433	976	1,281
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,326	3,248	9,780	9,638
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		998	1,003	2,973	2,971
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		544	503	1,571	1,497
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums		448	441	1,326	1,320
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		523	504	1,512	1,471
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		806	788	2,377	2,356
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		7	9	21	23
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums		250	249	729	712
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		591	573	1,721	1,684
Net investment income		57	59	174	177
Other revenues		5	5	16	14
Total segment revenues		653	637	1,911	1,875
Core income (loss)		136	165	444	540
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		540	529	1,584	1,555
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		243	239	717	708
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		249	245	728	714
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		48	45	139	133
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums		51	44	137	129
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		2,356	2,139	6,827	6,223
Net investment income		94	92	285	264
Fee income		5	5	13	12
Other revenues		14	16	45	47
Total segment revenues		2,469	2,252	7,170	6,546
Core income (loss)		77	163	178	410
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		2,191	1,988	6,362	5,780
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		1,192	1,026	3,431	2,936
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums		999	962	2,931	2,844
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums		165	151	465	443
Other [Member]
Segment reporting information
Other revenues		2	3	6	9
Core income (loss)		(65)	(60)	(188)	(183)
After-tax interest expense		$ 61	$ 57	$ 179	$ 177

[1] Total other-than-temporary impairment (OTTI) losses were $(5) million and $(4) million for the three months ended September 30, 2017 and 2016, respectively, and $(11) million and $(36) million for the nine months ended September 30, 2017 and 2016, respectively. Of total OTTI, credit losses of $(5) million and $(4) million for the three months ended September 30, 2017 and 2016, respectively, and $(12) million and $(26) million for the nine months ended September 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 September 30, 2017 and 2016, and $1 million and $(10) million for the nine months ended September 30, 2017 and 2016, 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.