Schedule III (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule III
Deferred acquisition costs	$ 3,306	$ 2,836	$ 2,542
Claims and claim adjustment expense reserves	61,627	58,649	56,907
Unearned premiums	20,872	18,240	16,469
Earned premiums	37,761	33,763	30,855
Net investment income	2,922	2,562	3,033
Claims and claim adjustment expenses	26,215	22,854	20,298
Amortization of deferred acquisition costs	6,226	5,515	5,043
Other operating expenses	5,552	5,161	5,017
Net written premiums	40,201	35,414	31,959
Reportable segments [Member]
Schedule III
Deferred acquisition costs	3,306	2,836	2,542
Claims and claim adjustment expense reserves	61,621	58,643	56,897
Unearned premiums	20,872	18,240	16,469
Earned premiums	37,761	33,763	30,855
Net investment income	2,922	2,562	3,033
Claims and claim adjustment expenses	26,215	22,854	20,298
Amortization of deferred acquisition costs	6,226	5,515	5,043
Other operating expenses	5,139	4,779	4,648
Net written premiums	40,201	35,414	31,959
Reportable segments [Member] | Business Insurance [Member]
Schedule III
Deferred acquisition costs	1,580	1,315	1,206
Claims and claim adjustment expense reserves	47,739	45,909	45,358
Unearned premiums	10,068	8,619	7,953
Earned premiums	19,144	17,095	15,734
Net investment income	2,085	1,864	2,265
Claims and claim adjustment expenses	12,696	10,907	10,398
Amortization of deferred acquisition costs	3,173	2,788	2,581
Other operating expenses	3,041	2,827	2,746
Net written premiums	20,430	17,635	16,092
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Schedule III
Deferred acquisition costs	477	430	375
Claims and claim adjustment expense reserves	4,945	4,482	4,247
Unearned premiums	2,861	2,679	2,360
Earned premiums	3,655	3,418	3,138
Net investment income	328	258	247
Claims and claim adjustment expenses	1,485	1,378	1,473
Amortization of deferred acquisition costs	673	625	570
Other operating expenses	681	590	532
Net written premiums	3,842	3,732	3,376
Reportable segments [Member] | Personal Insurance [Member]
Schedule III
Deferred acquisition costs	1,249	1,091	961
Claims and claim adjustment expense reserves	8,937	8,252	7,292
Unearned premiums	7,943	6,942	6,156
Earned premiums	14,962	13,250	11,983
Net investment income	509	440	521
Claims and claim adjustment expenses	12,034	10,569	8,427
Amortization of deferred acquisition costs	2,380	2,102	1,892
Other operating expenses	1,417	1,362	1,370
Net written premiums	15,929	14,047	12,491
Other [Member]
Schedule III
Deferred acquisition costs	0	0	0
Claims and claim adjustment expense reserves	6	6	10
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	413	382	369
Net written premiums	$ 0	$ 0	$ 0