Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Mar. 31, 2023 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 10,126	$ 8,854
Net investment income	846	663
Fee income	109	106
Other revenues	112	75
Net realized investment gains	35	6
Total revenues	11,228	9,704
Core income (loss)	1,096	970
Net realized investment gains, net of tax	27	5
Net income	1,123	975
Reportable Segments [Member]
Segment reporting information
Premiums	10,126	8,854
Net investment income	846	663
Fee income	109	106
Other revenues	112	75
Total segment revenues	11,193	9,698
Core income (loss)	1,179	1,046
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	5,160	4,477
Net investment income	609	473
Fee income	101	99
Other revenues	77	47
Total segment revenues	5,947	5,096
Core income (loss)	764	756
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	4,747	4,198
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	880	850
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	860	761
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	873	709
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	847	760
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	1,270	1,100
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	17	18
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	413	279
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	956	875
Net investment income	90	73
Fee income	0	0
Other revenues	9	5
Total segment revenues	1,055	953
Core income (loss)	195	207
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	822	758
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	426	398
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	340	306
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	56	54
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	134	117
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	4,010	3,502
Net investment income	147	117
Fee income	8	7
Other revenues	26	23
Total segment revenues	4,191	3,649
Core income (loss)	220	83
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,846	3,348
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,874	1,624
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,972	1,724
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	164	154
Other [Member]
Segment reporting information
Core income (loss)	(83)	(76)
After-tax interest expense	$ 77	$ 70