Schedule III (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule III
Deferred acquisition costs	$ 2,542	$ 2,358	$ 2,273
Claims and claim adjustment expense reserves	56,907	54,521	51,849
Unearned premiums	16,469	15,222	14,604
Earned premiums	30,855	29,044	28,272
Net investment income	3,033	2,227	2,468
Claims and claim adjustment expenses	20,298	19,123	19,133
Amortization of deferred acquisition costs	5,043	4,773	4,601
Other operating expenses	5,017	4,848	4,709
Net written premiums	31,959	29,732	29,151
Reportable segments [Member]
Schedule III
Deferred acquisition costs	2,542	2,358	2,273
Claims and claim adjustment expense reserves	56,897	54,510	51,836
Unearned premiums	16,469	15,222	14,604
Earned premiums	30,855	29,044	28,272
Net investment income	3,033	2,227	2,468
Claims and claim adjustment expenses	20,298	19,123	19,133
Amortization of deferred acquisition costs	5,043	4,773	4,601
Other operating expenses	4,648	4,478	4,330
Net written premiums	31,959	29,732	29,151
Reportable segments [Member] | Business Insurance [Member]
Schedule III
Deferred acquisition costs	1,206	1,152	1,161
Claims and claim adjustment expense reserves	45,358	44,162	42,252
Unearned premiums	7,953	7,556	7,540
Earned premiums	15,734	15,294	15,300
Net investment income	2,265	1,633	1,816
Claims and claim adjustment expenses	10,398	10,804	10,963
Amortization of deferred acquisition costs	2,581	2,518	2,503
Other operating expenses	2,746	2,664	2,627
Net written premiums	16,092	15,431	15,629
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Schedule III
Deferred acquisition costs	375	328	309
Claims and claim adjustment expense reserves	4,247	3,814	3,316
Unearned premiums	2,360	2,020	1,845
Earned premiums	3,138	2,823	2,565
Net investment income	247	213	233
Claims and claim adjustment expenses	1,473	1,464	1,094
Amortization of deferred acquisition costs	570	519	478
Other operating expenses	532	500	483
Net written premiums	3,376	2,951	2,739
Reportable segments [Member] | Personal Insurance [Member]
Schedule III
Deferred acquisition costs	961	878	803
Claims and claim adjustment expense reserves	7,292	6,534	6,268
Unearned premiums	6,156	5,646	5,219
Earned premiums	11,983	10,927	10,407
Net investment income	521	381	419
Claims and claim adjustment expenses	8,427	6,855	7,076
Amortization of deferred acquisition costs	1,892	1,736	1,620
Other operating expenses	1,370	1,314	1,220
Net written premiums	12,491	11,350	10,783
Other [Member]
Schedule III
Deferred acquisition costs	0	0	0
Claims and claim adjustment expense reserves	10	11	13
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	369	370	379
Net written premiums	$ 0	$ 0	$ 0