Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 9,216	$ 8,317	$ 18,070	$ 16,331
Net investment income	712	707	1,375	1,344
Fee income	106	100	212	203
Other revenues	99	107	174	185
Net realized investment losses	(35)	(95)	(29)	(118)
Total revenues	10,098	9,136	19,802	17,945
Core income (loss)	15	625	985	1,662
Net realized investment losses, net of tax	(29)	(74)	(24)	(93)
Net income (loss)	(14)	551	961	1,569
Reportable Segments [Member]
Segment reporting information
Premiums	9,216	8,317	18,070	16,331
Net investment income	712	707	1,375	1,344
Fee income	106	100	212	203
Other revenues	99	107	174	185
Total segment revenues	10,133	9,231	19,831	18,063
Core income (loss)	94	701	1,140	1,812
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	4,644	4,218	9,121	8,289
Net investment income	509	521	982	989
Fee income	98	93	197	189
Other revenues	67	85	114	138
Total segment revenues	5,318	4,917	10,414	9,605
Core income (loss)	402	666	1,158	1,335
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	4,325	3,963	8,523	7,786
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	858	864	1,708	1,694
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	798	735	1,559	1,448
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	760	638	1,469	1,246
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	748	700	1,508	1,388
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	1,145	1,010	2,245	1,977
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	16	16	34	33
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	319	255	598	503
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	911	851	1,786	1,671
Net investment income	78	64	151	123
Fee income	0	0	0	0
Other revenues	7	4	12	8
Total segment revenues	996	919	1,949	1,802
Core income (loss)	230	228	437	445
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	788	734	1,546	1,436
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	406	385	804	758
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	325	295	631	569
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	57	54	111	109
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	123	117	240	235
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,661	3,248	7,163	6,371
Net investment income	125	122	242	232
Fee income	8	7	15	14
Other revenues	25	18	48	39
Total segment revenues	3,819	3,395	7,468	6,656
Core income (loss)	(538)	(193)	(455)	32
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,502	3,081	6,850	6,035
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,688	1,511	3,312	2,969
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,814	1,570	3,538	3,066
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	159	167	313	336
Other [Member]
Segment reporting information
Core income (loss)	(79)	(76)	(155)	(150)
After-tax interest expense	$ 72	$ 69	$ 142	$ 138