Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment reporting information
Premiums		$ 6,882	$ 6,523	$ 20,114	$ 19,057
Net investment income		646	588	1,844	1,796
Fee income		109	113	324	342
Other revenues		57	40	150	110
Net realized investment gains	[1]	29	61	54	146
Total revenues		7,723	7,325	22,486	21,451
Core income (loss)		687	253	1,859	1,410
Net realized investment gains, net of tax		22	40	43	95
Net income		709	293	1,902	1,505
Reportable Segments [Member]
Segment reporting information
Premiums		6,882	6,523	20,114	19,057
Net investment income		646	588	1,844	1,796
Fee income		109	113	324	342
Other revenues		55	38	148	104
Total segment revenues		7,692	7,262	22,430	21,299
Core income (loss)		759	318	2,085	1,598
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums		3,743	3,576	10,952	10,509
Net investment income		482	437	1,368	1,337
Fee income		103	108	309	329
Other revenues		33	19	84	43
Total segment revenues		4,361	4,140	12,713	12,218
Core income (loss)		410	105	1,247	976
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,480	3,326	10,160	9,780
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		988	998	2,932	2,973
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		611	544	1,760	1,571
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums		466	448	1,357	1,326
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		558	523	1,614	1,512
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		850	806	2,477	2,377
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		7	7	20	21
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums		263	250	792	729
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		617	591	1,800	1,721
Net investment income		57	57	172	174
Other revenues		5	5	16	16
Total segment revenues		679	653	1,988	1,911
Core income (loss)		196	136	573	444
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		566	540	1,651	1,584
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		255	243	745	717
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		261	249	760	728
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		50	48	146	139
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums		51	51	149	137
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		2,522	2,356	7,362	6,827
Net investment income		107	94	304	285
Fee income		6	5	15	13
Other revenues		17	14	48	45
Total segment revenues		2,652	2,469	7,729	7,170
Core income (loss)		153	77	265	178
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		2,348	2,191	6,851	6,362
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		1,297	1,192	3,783	3,431
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums		1,051	999	3,068	2,931
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums		174	165	511	465
Other [Member]
Segment reporting information
Other revenues		2	2	2	6
Core income (loss)		(72)	(65)	(226)	(188)
After-tax interest expense		$ 68	$ 61	$ 209	$ 179

[1] Total other-than-temporary impairment (OTTI) losses were $0 million and $(5) million for the three months ended September 30, 2018 and 2017, respectively, and $(1) million and $(11) million for the nine months ended September 30, 2018 and 2017, respectively. Of total OTTI, credit losses of $0 million and $(5) million for the three months ended September 30, 2018 and 2017, respectively, and $(1) million and $(12) million for the nine months ended September 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 September 30, 2018 and 2017, respectively, and $0 million and $1 million for the nine months ended September 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.