Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 9,718	$ 8,615	$ 27,788	$ 24,946
Net investment income	769	593	2,144	1,937
Fee income	112	104	324	307
Other revenues	101	84	275	269
Net realized investment losses	(65)	(93)	(94)	(211)
Total revenues	10,635	9,303	30,437	27,248
Core income (loss)	454	526	1,439	2,188
Net realized investment losses, net of tax	(50)	(72)	(74)	(165)
Net income	404	454	1,365	2,023
Reportable Segments [Member]
Segment reporting information
Premiums	9,718	8,615	27,788	24,946
Net investment income	769	593	2,144	1,937
Fee income	112	104	324	307
Other revenues	101	84	275	269
Total segment revenues	10,700	9,396	30,531	27,459
Core income (loss)	540	602	1,680	2,414
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	4,956	4,353	14,077	12,642
Net investment income	551	426	1,533	1,415
Fee income	102	96	299	285
Other revenues	71	56	185	194
Total segment revenues	5,680	4,931	16,094	14,536
Core income (loss)	468	471	1,626	1,806
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	4,556	4,098	13,079	11,884
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	876	872	2,584	2,566
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	816	754	2,375	2,202
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	828	668	2,297	1,914
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	804	728	2,312	2,116
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	1,211	1,057	3,456	3,034
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	21	19	55	52
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	400	255	998	758
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	935	877	2,721	2,548
Net investment income	86	65	237	188
Fee income	0	0	0	0
Other revenues	6	6	18	14
Total segment revenues	1,027	948	2,976	2,750
Core income (loss)	265	242	702	687
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	804	760	2,350	2,196
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	414	396	1,218	1,154
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	333	307	964	876
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	57	57	168	166
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	131	117	371	352
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,827	3,385	10,990	9,756
Net investment income	132	102	374	334
Fee income	10	8	25	22
Other revenues	24	22	72	61
Total segment revenues	3,993	3,517	11,461	10,173
Core income (loss)	(193)	(111)	(648)	(79)
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,668	3,223	10,518	9,258
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,772	1,575	5,084	4,544
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,896	1,648	5,434	4,714
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	159	162	472	498
Other [Member]
Segment reporting information
Core income (loss)	(86)	(76)	(241)	(226)
After-tax interest expense	$ 78	$ 70	$ 220	$ 208