Schedule III (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Schedule III
Deferred acquisition costs	$ 3,494	$ 3,306	$ 2,836
Claims and claim adjustment expense reserves	64,093	61,627	58,649
Unearned premiums	22,289	20,872	18,240
Earned premiums	41,941	37,761	33,763
Net investment income	3,590	2,922	2,562
Claims and claim adjustment expenses	27,059	26,215	22,854
Amortization of deferred acquisition costs	6,973	6,226	5,515
Other operating expenses	6,211	5,552	5,161
Net written premiums	43,356	40,201	35,414
Reportable segments [Member]
Schedule III
Deferred acquisition costs	3,494	3,306	2,836
Claims and claim adjustment expense reserves	64,088	61,621	58,643
Unearned premiums	22,289	20,872	18,240
Earned premiums	41,941	37,761	33,763
Net investment income	3,590	2,922	2,562
Claims and claim adjustment expenses	27,059	26,215	22,854
Amortization of deferred acquisition costs	6,973	6,226	5,515
Other operating expenses	5,775	5,139	4,779
Net written premiums	43,356	40,201	35,414
Reportable segments [Member] | Business Insurance [Member]
Schedule III
Deferred acquisition costs	1,698	1,580	1,315
Claims and claim adjustment expense reserves	49,731	47,739	45,909
Unearned premiums	10,831	10,068	8,619
Earned premiums	21,345	19,144	17,095
Net investment income	2,560	2,085	1,864
Claims and claim adjustment expenses	13,679	12,696	10,907
Amortization of deferred acquisition costs	3,588	3,173	2,788
Other operating expenses	3,303	3,041	2,827
Net written premiums	22,078	20,430	17,635
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Schedule III
Deferred acquisition costs	502	477	430
Claims and claim adjustment expense reserves	5,365	4,945	4,482
Unearned premiums	3,012	2,861	2,679
Earned premiums	3,958	3,655	3,418
Net investment income	390	328	258
Claims and claim adjustment expenses	1,774	1,485	1,378
Amortization of deferred acquisition costs	756	673	625
Other operating expenses	832	681	590
Net written premiums	4,109	3,842	3,732
Reportable segments [Member] | Personal Insurance [Member]
Schedule III
Deferred acquisition costs	1,294	1,249	1,091
Claims and claim adjustment expense reserves	8,992	8,937	8,252
Unearned premiums	8,446	7,943	6,942
Earned premiums	16,638	14,962	13,250
Net investment income	640	509	440
Claims and claim adjustment expenses	11,606	12,034	10,569
Amortization of deferred acquisition costs	2,629	2,380	2,102
Other operating expenses	1,640	1,417	1,362
Net written premiums	17,169	15,929	14,047
Other [Member]
Schedule III
Deferred acquisition costs	0	0	0
Claims and claim adjustment expense reserves	5	6	6
Unearned premiums	0	0	0
Earned premiums	0	0	0
Net investment income	0	0	0
Claims and claim adjustment expenses	0	0	0
Amortization of deferred acquisition costs	0	0	0
Other operating expenses	436	413	382
Net written premiums	$ 0	$ 0	$ 0