Segment Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 41,941	$ 37,761	$ 33,763
Net investment income	3,590	2,922	2,562
Fee income	473	433	412
Other revenues	449	353	351
Claims and claim adjustment expenses	27,059	26,215	22,854
Amortization of deferred acquisition costs	6,973	6,226	5,515
General and administrative expenses	5,819	5,176	4,810
Income tax expense (benefit)	1,181	380	512
Net realized investment losses	(30)	(105)	(204)
Total revenues	46,423	41,364	36,884
Core income (loss)	5,025	3,072	2,998
Net realized investment losses, net of tax	(26)	(81)	(156)
Net income	4,999	2,991	2,842
Domestic [Member]
Segment reporting information
Total revenues	43,924	39,086	34,822
International [Member]
Segment reporting information
Total revenues	2,499	2,278	2,062
Reportable segments [Member]
Segment reporting information
Premiums	41,941	37,761	33,763
Net investment income	3,590	2,922	2,562
Fee income	473	433	412
Other revenues	449	353	351
Total segment revenues	46,453	41,469	37,088
Claims and claim adjustment expenses	27,059	26,215	22,854
Amortization of deferred acquisition costs	6,973	6,226	5,515
General and administrative expenses	5,775	5,139	4,779
Income tax expense (benefit)	1,276	492	641
Core income (loss)	5,370	3,397	3,299
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	21,345	19,144	17,095
Net investment income	2,560	2,085	1,864
Fee income	430	400	382
Other revenues	322	232	248
Total segment revenues	24,657	21,861	19,589
Claims and claim adjustment expenses	13,679	12,696	10,907
Amortization of deferred acquisition costs	3,588	3,173	2,788
General and administrative expenses	3,303	3,041	2,827
Income tax expense (benefit)	781	368	536
Core income (loss)	3,306	2,583	2,531
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	19,482	17,744	16,072
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	3,470	3,467	3,425
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	3,590	3,215	2,976
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	3,616	3,154	2,611
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	3,464	3,146	2,875
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	5,269	4,686	4,109
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	73	76	76
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	1,863	1,400	1,023
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	3,958	3,655	3,418
Net investment income	390	328	258
Fee income	0	0	0
Other revenues	30	25	20
Total segment revenues	4,378	4,008	3,696
Claims and claim adjustment expenses	1,774	1,485	1,378
Amortization of deferred acquisition costs	756	673	625
General and administrative expenses	832	681	590
Income tax expense (benefit)	201	227	195
Core income (loss)	815	942	908
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,425	3,154	2,951
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	1,778	1,639	1,556
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	1,416	1,290	1,173
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	231	225	222
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	533	501	467
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	16,638	14,962	13,250
Net investment income	640	509	440
Fee income	43	33	30
Other revenues	97	96	83
Total segment revenues	17,418	15,600	13,803
Claims and claim adjustment expenses	11,606	12,034	10,569
Amortization of deferred acquisition costs	2,629	2,380	2,102
General and administrative expenses	1,640	1,417	1,362
Income tax expense (benefit)	294	(103)	(90)
Core income (loss)	1,249	(128)	(140)
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	15,975	14,327	12,596
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	7,767	6,923	6,170
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	8,208	7,404	6,426
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	663	635	654
Other [Member]
Segment reporting information
Core income (loss)	(345)	(325)	(301)
After-tax interest expense	$ 310	$ 297	$ 277