Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 8,014	$ 7,386
Net investment income	637	701
Fee income	103	101
Other revenues	78	81
Net realized investment gains (losses)	(23)	44
Total revenues	8,809	8,313
Core income (loss)	1,037	699
Net realized investment gains (losses), net of tax	(19)	34
Net income	1,018	733
Reportable Segments [Member]
Segment reporting information
Premiums	8,014	7,386
Net investment income	637	701
Fee income	103	101
Other revenues	78	81
Total segment revenues	8,832	8,269
Core income (loss)	1,111	768
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	4,071	3,799
Net investment income	468	523
Fee income	96	95
Other revenues	53	53
Total segment revenues	4,688	4,470
Core income (loss)	669	317
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,823	3,528
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	830	811
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	713	695
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	608	531
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	688	607
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	967	870
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	17	14
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	248	271
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	820	743
Net investment income	59	59
Fee income	0	0
Other revenues	4	5
Total segment revenues	883	807
Core income (loss)	217	137
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	702	653
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	373	331
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	274	266
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	55	56
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	118	90
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,123	2,844
Net investment income	110	119
Fee income	7	6
Other revenues	21	23
Total segment revenues	3,261	2,992
Core income (loss)	225	314
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,954	2,678
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,458	1,370
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,496	1,308
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	169	166
Other [Member]
Segment reporting information
Core income (loss)	(74)	(69)
After-tax interest expense	$ 69	$ 65