Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 8,615	$ 7,829	$ 24,946	$ 22,831
Net investment income	593	771	1,937	2,290
Fee income	104	97	307	302
Other revenues	84	100	269	269
Net realized investment gains (losses)	(93)	8	(211)	113
Total revenues	9,303	8,805	27,248	25,805
Core income (loss)	526	655	2,188	2,233
Net realized investment gains (losses), net of tax	(72)	7	(165)	88
Impact of changes in tax laws and/or tax rates	0	0	0	8
Net income	454	662	2,023	2,329
Reportable Segments [Member]
Segment reporting information
Premiums	8,615	7,829	24,946	22,831
Net investment income	593	771	1,937	2,290
Fee income	104	97	307	302
Other revenues	84	100	269	269
Total segment revenues	9,396	8,797	27,459	25,692
Core income (loss)	602	730	2,414	2,449
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	4,353	3,970	12,642	11,649
Net investment income	426	575	1,415	1,713
Fee income	96	90	285	282
Other revenues	56	69	194	179
Total segment revenues	4,931	4,704	14,536	13,823
Core income (loss)	471	558	1,806	1,518
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	4,098	3,707	11,884	10,841
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	872	789	2,566	2,410
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	754	718	2,202	2,122
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	668	582	1,914	1,669
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	728	662	2,116	1,887
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	1,057	939	3,034	2,708
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	19	17	52	45
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	255	263	758	808
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	877	806	2,548	2,325
Net investment income	65	63	188	186
Fee income	0	0	0	0
Other revenues	6	5	14	17
Total segment revenues	948	874	2,750	2,528
Core income (loss)	242	174	687	498
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	760	696	2,196	2,027
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	396	365	1,154	1,043
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	307	277	876	819
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	57	54	166	165
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	117	110	352	298
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,385	3,053	9,756	8,857
Net investment income	102	133	334	391
Fee income	8	7	22	20
Other revenues	22	26	61	73
Total segment revenues	3,517	3,219	10,173	9,341
Core income (loss)	(111)	(2)	(79)	433
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,223	2,882	9,258	8,342
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,575	1,445	4,544	4,220
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,648	1,437	4,714	4,122
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	162	171	498	515
Other [Member]
Segment reporting information
Core income (loss)	(76)	(75)	(226)	(216)
After-tax interest expense	$ 70	$ 69	$ 208	$ 199