Segment Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) segment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 33,763	$ 30,855	$ 29,044
Net investment income	2,562	3,033	2,227
Fee income	412	402	429
Other revenues	351	355	279
Income tax expense (benefit)	512	796	540
Net realized investment gains (losses)	(204)	171	2
Total revenues	36,884	34,816	31,981
Core income (loss)	2,998	3,522	2,686
Net realized investment gains (losses), net of tax	(156)	132	11
Impact of changes in tax laws and/or tax rates	0	8	0
Net income	2,842	3,662	2,697
Domestic [Member]
Segment reporting information
Total revenues	34,822	32,596	30,123
International [Member]
Segment reporting information
Total revenues	2,062	2,220	1,858
Reportable segments [Member]
Segment reporting information
Premiums	33,763	30,855	29,044
Net investment income	2,562	3,033	2,227
Fee income	412	402	429
Other revenues	351	355	279
Total segment revenues	37,088	34,645	31,979
Amortization and depreciation	6,334	5,907	5,556
Income tax expense (benefit)	641	843	628
Core income (loss)	3,299	3,813	2,977
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	17,095	15,734	15,294
Net investment income	1,864	2,265	1,633
Fee income	382	375	405
Other revenues	248	235	176
Total segment revenues	19,589	18,609	17,508
Amortization and depreciation	3,344	3,180	3,069
Income tax expense (benefit)	536	499	213
Core income (loss)	2,531	2,385	1,309
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	16,072	14,665	14,233
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	3,425	3,227	3,378
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	2,976	2,855	2,761
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	2,611	2,275	2,087
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	2,875	2,576	2,401
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	4,109	3,667	3,552
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	76	65	54
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	1,023	1,069	1,061
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	3,418	3,138	2,823
Net investment income	258	247	213
Fee income	0	0	0
Other revenues	20	23	27
Total segment revenues	3,696	3,408	3,063
Amortization and depreciation	697	643	579
Income tax expense (benefit)	195	165	107
Core income (loss)	908	668	473
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,951	2,726	2,531
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	1,556	1,415	1,219
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	1,173	1,091	1,075
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	222	220	237
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	467	412	292
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	13,250	11,983	10,927
Net investment income	440	521	381
Fee income	30	27	24
Other revenues	83	97	76
Total segment revenues	13,803	12,628	11,408
Amortization and depreciation	2,293	2,084	1,908
Income tax expense (benefit)	(90)	179	308
Core income (loss)	(140)	760	1,195
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	12,596	11,295	10,268
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	6,170	5,687	5,280
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	6,426	5,608	4,988
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	654	688	659
Other [Member]
Segment reporting information
Core income (loss)	(301)	(291)	(291)
After-tax interest expense	$ 277	$ 269	$ 268