Schedule III (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule III
Deferred acquisition costs	$ 2,120	$ 2,025	$ 1,923
Claims and claim adjustment expense reserves	50,668	49,650	47,949
Unearned premiums	13,555	12,915	12,329
Earned premiums	27,059	25,683	24,534
Net investment income	2,474	2,397	2,302
Claims and claim adjustment expenses	18,291	17,467	15,070
Amortization of deferred acquisition costs	4,381	4,166	3,985
Other operating expenses	4,649	4,539	4,517
Net written premiums	27,708	26,219	24,958
Reportable segments [Member]
Schedule III
Deferred acquisition costs	2,120	2,025	1,923
Claims and claim adjustment expense reserves	50,653	49,633	47,929
Unearned premiums	13,555	12,915	12,329
Earned premiums	27,059	25,683	24,534
Net investment income	2,474	2,397	2,302
Claims and claim adjustment expenses	18,291	17,467	15,070
Amortization of deferred acquisition costs	4,381	4,166	3,985
Other operating expenses	4,267	4,138	4,123
Net written premiums	27,708	26,219	24,958
Reportable segments [Member] | Business Insurance [Member]
Schedule III
Deferred acquisition costs	1,102	1,060	1,026
Claims and claim adjustment expense reserves	41,132	40,352	39,555
Unearned premiums	7,112	6,857	6,725
Earned premiums	14,722	14,146	13,855
Net investment income	1,833	1,786	1,701
Claims and claim adjustment expenses	10,171	9,521	8,753
Amortization of deferred acquisition costs	2,388	2,286	2,221
Other operating expenses	2,623	2,563	2,554
Net written premiums	14,956	14,270	13,900
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Schedule III
Deferred acquisition costs	277	258	246
Claims and claim adjustment expense reserves	3,255	3,421	3,323
Unearned premiums	1,619	1,515	1,444
Earned premiums	2,420	2,307	2,260
Net investment income	233	228	239
Claims and claim adjustment expenses	772	899	633
Amortization of deferred acquisition costs	454	432	421
Other operating expenses	459	464	445
Net written premiums	2,528	2,359	2,271
Reportable segments [Member] | Personal Insurance [Member]
Schedule III
Deferred acquisition costs	741	707	651
Claims and claim adjustment expense reserves	6,266	5,860	5,051
Unearned premiums	4,824	4,543	4,160
Earned premiums	9,917	9,230	8,419
Net investment income	408	383	362
Claims and claim adjustment expenses	7,348	7,047	5,684
Amortization of deferred acquisition costs	1,539	1,448	1,343
Other operating expenses	1,185	1,111	1,124
Net written premiums	10,224	9,590	8,787
Other [Member]
Schedule III
Claims and claim adjustment expense reserves	15	17	20
Other operating expenses	$ 382	$ 401	$ 394