Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) segment	Mar. 31, 2022 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 8,854	$ 8,014
Net investment income	663	637
Fee income	106	103
Other revenues	75	78
Net realized investment gains (losses)	6	(23)
Total revenues	9,704	8,809
Core income (loss)	970	1,037
Net realized investment gains (losses), net of tax	5	(19)
Net income	975	1,018
Reportable Segments [Member]
Segment reporting information
Premiums	8,854	8,014
Net investment income	663	637
Fee income	106	103
Other revenues	75	78
Total segment revenues	9,698	8,832
Core income (loss)	1,046	1,111
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	4,477	4,071
Net investment income	473	468
Fee income	99	96
Other revenues	47	53
Total segment revenues	5,096	4,688
Core income (loss)	756	669
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	4,198	3,823
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	850	830
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	761	713
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	709	608
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	760	688
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	1,100	967
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	18	17
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	279	248
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	875	820
Net investment income	73	59
Fee income	0	0
Other revenues	5	4
Total segment revenues	953	883
Core income (loss)	207	217
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	758	702
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	398	373
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	306	274
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	54	55
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	117	118
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,502	3,123
Net investment income	117	110
Fee income	7	7
Other revenues	23	21
Total segment revenues	3,649	3,261
Core income (loss)	83	225
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,348	2,954
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,624	1,458
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,724	1,496
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	154	169
Other [Member]
Segment reporting information
Core income (loss)	(76)	(74)
After-tax interest expense	$ 70	$ 69