Segment Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 30,855	$ 29,044	$ 28,272
Net investment income	3,033	2,227	2,468
Fee income	402	429	459
Other revenues	355	279	269
Income tax expense	796	540	516
Net realized investment gains	171	2	113
Total revenues	34,816	31,981	31,581
Core income (loss)	3,522	2,686	2,537
Net realized investment gains, net of tax	132	11	85
Impact of changes in tax laws and/or tax rates	8	0	0
Net income	3,662	2,697	2,622
Domestic [Member]
Segment reporting information
Total revenues	32,596	30,123	29,638
International [Member]
Segment reporting information
Total revenues	2,220	1,858	1,943
Reportable segments [Member]
Segment reporting information
Premiums	30,855	29,044	28,272
Net investment income	3,033	2,227	2,468
Fee income	402	429	459
Other revenues	355	279	268
Total segment revenues	34,645	31,979	31,467
Amortization and depreciation	5,907	5,556	5,357
Income tax expense	843	628	569
Core income (loss)	3,813	2,977	2,834
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	15,734	15,294	15,300
Net investment income	2,265	1,633	1,816
Fee income	375	405	437
Other revenues	235	176	155
Total segment revenues	18,609	17,508	17,708
Amortization and depreciation	3,180	3,069	3,037
Income tax expense	499	213	223
Core income (loss)	2,385	1,309	1,392
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	14,665	14,233	14,233
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	3,227	3,378	3,829
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	2,855	2,761	2,632
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	2,275	2,087	1,937
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	2,576	2,401	2,342
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	3,667	3,552	3,453
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	65	54	40
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	1,069	1,061	1,067
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	3,138	2,823	2,565
Net investment income	247	213	233
Fee income	0	0	0
Other revenues	23	27	26
Total segment revenues	3,408	3,063	2,824
Amortization and depreciation	643	579	533
Income tax expense	165	107	151
Core income (loss)	668	473	618
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,726	2,531	2,334
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	1,415	1,219	1,082
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	1,091	1,075	1,036
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	220	237	216
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	412	292	231
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	11,983	10,927	10,407
Net investment income	521	381	419
Fee income	27	24	22
Other revenues	97	76	87
Total segment revenues	12,628	11,408	10,935
Amortization and depreciation	2,084	1,908	1,787
Income tax expense	179	308	195
Core income (loss)	760	1,195	824
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	11,295	10,268	9,704
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	5,687	5,280	5,311
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	5,608	4,988	4,393
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	688	659	703
Other [Member]
Segment reporting information
Other revenues	0	0	1
Core income (loss)	(291)	(291)	(297)
After-tax interest expense	$ 269	$ 268	$ 272