Schedule III (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule III
Deferred acquisition costs	$ 2,358	$ 2,273	$ 2,120
Claims and claim adjustment expense reserves	54,521	51,849	50,668
Unearned premiums	15,222	14,604	13,555
Earned premiums	29,044	28,272	27,059
Net investment income	2,227	2,468	2,474
Claims and claim adjustment expenses	19,123	19,133	18,291
Amortization of deferred acquisition costs	4,773	4,601	4,381
Other operating expenses	4,848	4,709	4,649
Net written premiums	29,732	29,151	27,708
Reportable segments [Member]
Schedule III
Deferred acquisition costs	2,358	2,273	2,120
Claims and claim adjustment expense reserves	54,510	51,836	50,653
Unearned premiums	15,222	14,604	13,555
Earned premiums	29,044	28,272	27,059
Net investment income	2,227	2,468	2,474
Claims and claim adjustment expenses	19,123	19,133	18,291
Amortization of deferred acquisition costs	4,773	4,601	4,381
Other operating expenses	4,478	4,330	4,267
Net written premiums	29,732	29,151	27,708
Reportable segments [Member] | Business Insurance [Member]
Schedule III
Deferred acquisition costs	1,152	1,161	1,102
Claims and claim adjustment expense reserves	44,162	42,252	41,132
Unearned premiums	7,556	7,540	7,112
Earned premiums	15,294	15,300	14,722
Net investment income	1,633	1,816	1,833
Claims and claim adjustment expenses	10,804	10,963	10,171
Amortization of deferred acquisition costs	2,518	2,503	2,388
Other operating expenses	2,664	2,627	2,623
Net written premiums	15,431	15,629	14,956
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Schedule III
Deferred acquisition costs	328	309	277
Claims and claim adjustment expense reserves	3,814	3,316	3,255
Unearned premiums	2,020	1,845	1,619
Earned premiums	2,823	2,565	2,420
Net investment income	213	233	233
Claims and claim adjustment expenses	1,464	1,094	772
Amortization of deferred acquisition costs	519	478	454
Other operating expenses	500	483	459
Net written premiums	2,951	2,739	2,528
Reportable segments [Member] | Personal Insurance [Member]
Schedule III
Deferred acquisition costs	878	803	741
Claims and claim adjustment expense reserves	6,534	6,268	6,266
Unearned premiums	5,646	5,219	4,824
Earned premiums	10,927	10,407	9,917
Net investment income	381	419	408
Claims and claim adjustment expenses	6,855	7,076	7,348
Amortization of deferred acquisition costs	1,736	1,620	1,539
Other operating expenses	1,314	1,220	1,185
Net written premiums	11,350	10,783	10,224
Other [Member]
Schedule III
Deferred acquisition costs	0	0	0
Claims and claim adjustment expense reserves	11	13	15
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	370	379	382
Net written premiums	$ 0	$ 0	$ 0