Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) segment	Jun. 30, 2021 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 8,317	$ 7,616	$ 16,331	$ 15,002
Net investment income	707	818	1,344	1,519
Fee income	100	104	203	205
Other revenues	107	88	185	169
Net realized investment gains (losses)	(95)	61	(118)	105
Total revenues	9,136	8,687	17,945	17,000
Core income (loss)	625	879	1,662	1,578
Net realized investment gains (losses), net of tax	(74)	47	(93)	81
Impact of changes in tax laws and/or tax rates	0	8	0	8
Net income	551	934	1,569	1,667
Reportable Segments [Member]
Segment reporting information
Premiums	8,317	7,616	16,331	15,002
Net investment income	707	818	1,344	1,519
Fee income	100	104	203	205
Other revenues	107	88	185	169
Total segment revenues	9,231	8,626	18,063	16,895
Core income (loss)	701	951	1,812	1,719
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	4,218	3,880	8,289	7,679
Net investment income	521	615	989	1,138
Fee income	93	97	189	192
Other revenues	85	57	138	110
Total segment revenues	4,917	4,649	9,605	9,119
Core income (loss)	666	643	1,335	960
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,963	3,606	7,786	7,134
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	864	810	1,694	1,621
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	735	709	1,448	1,404
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	638	556	1,246	1,087
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	700	618	1,388	1,225
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	1,010	899	1,977	1,769
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	16	14	33	28
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	255	274	503	545
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	851	776	1,671	1,519
Net investment income	64	64	123	123
Fee income	0	0	0	0
Other revenues	4	7	8	12
Total segment revenues	919	847	1,802	1,654
Core income (loss)	228	187	445	324
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	734	678	1,436	1,331
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	385	347	758	678
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	295	276	569	542
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	54	55	109	111
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	117	98	235	188
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,248	2,960	6,371	5,804
Net investment income	122	139	232	258
Fee income	7	7	14	13
Other revenues	18	24	39	47
Total segment revenues	3,395	3,130	6,656	6,122
Core income (loss)	(193)	121	32	435
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,081	2,782	6,035	5,460
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,511	1,405	2,969	2,775
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,570	1,377	3,066	2,685
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	167	178	336	344
Other [Member]
Segment reporting information
Core income (loss)	(76)	(72)	(150)	(141)
After-tax interest expense	$ 69	$ 65	$ 138	$ 130