Segment Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 37,761	$ 33,763	$ 30,855
Net investment income	2,922	2,562	3,033
Fee income	433	412	402
Other revenues	353	351	355
Income tax expense (benefit)	380	512	796
Net realized investment gains (losses)	(105)	(204)	171
Total revenues	41,364	36,884	34,816
Core income (loss)	3,072	2,998	3,522
Net realized investment gains (losses), net of tax	(81)	(156)	132
Impact of changes in tax laws and/or tax rates	0	0	8
Net income	2,991	2,842	3,662
Domestic [Member]
Segment reporting information
Total revenues	39,086	34,822	32,596
International [Member]
Segment reporting information
Total revenues	2,278	2,062	2,220
Reportable segments [Member]
Segment reporting information
Premiums	37,761	33,763	30,855
Net investment income	2,922	2,562	3,033
Fee income	433	412	402
Other revenues	353	351	355
Total segment revenues	41,469	37,088	34,645
Amortization and depreciation	6,942	6,334	5,907
Income tax expense (benefit)	492	641	843
Core income (loss)	3,397	3,299	3,813
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	19,144	17,095	15,734
Net investment income	2,085	1,864	2,265
Fee income	400	382	375
Other revenues	232	248	235
Total segment revenues	21,861	19,589	18,609
Amortization and depreciation	3,640	3,344	3,180
Income tax expense (benefit)	368	536	499
Core income (loss)	2,583	2,531	2,385
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	17,744	16,072	14,665
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	3,467	3,425	3,227
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	3,215	2,976	2,855
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	3,154	2,611	2,275
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	3,146	2,875	2,576
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	4,686	4,109	3,667
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	76	76	65
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	1,400	1,023	1,069
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	3,655	3,418	3,138
Net investment income	328	258	247
Fee income	0	0	0
Other revenues	25	20	23
Total segment revenues	4,008	3,696	3,408
Amortization and depreciation	744	697	643
Income tax expense (benefit)	227	195	165
Core income (loss)	942	908	668
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,154	2,951	2,726
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	1,639	1,556	1,415
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	1,290	1,173	1,091
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	225	222	220
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	501	467	412
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	14,962	13,250	11,983
Net investment income	509	440	521
Fee income	33	30	27
Other revenues	96	83	97
Total segment revenues	15,600	13,803	12,628
Amortization and depreciation	2,558	2,293	2,084
Income tax expense (benefit)	(103)	(90)	179
Core income (loss)	(128)	(140)	760
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	14,327	12,596	11,295
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	6,923	6,170	5,687
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	7,404	6,426	5,608
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	635	654	688
Other [Member]
Segment reporting information
Core income (loss)	(325)	(301)	(291)
After-tax interest expense	$ 297	$ 277	$ 269