Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	$ 682	$ 602	$ 542
Reserves for unpaid claims and claim adjustment expenses	8,336	7,229	6,677
Unearned premiums	3,688	3,270	2,959
Earned premiums	6,924	6,184	5,691
Investment income	509	457	431
Loss and loss expenses incurred related to current accident year	4,875	4,024	3,968
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(159)	(428)	(131)
Amortization of deferred policy acquisition cost	1,303	1,151	1,057
Paid loss and loss expenses	3,687	3,094	3,183
Net written premiums	7,307	6,479	5,864
Commercial Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	372	345	317
Reserves for unpaid claims and claim adjustment expenses	5,568	5,007	4,881
Unearned premiums	2,021	1,857	1,714
Earned premiums	4,024	3,674	3,476
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,837	2,293	2,431
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(76)	(353)	(95)
Amortization of deferred policy acquisition cost	769	703	664
Paid loss and loss expenses	2,211	1,806	1,969
Net written premiums	4,159	3,811	3,534
Personal Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	168	145	136
Reserves for unpaid claims and claim adjustment expenses	916	814	739
Unearned premiums	994	850	797
Earned premiums	1,689	1,542	1,463
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,227	1,042	995
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(61)	(50)	(18)
Amortization of deferred policy acquisition cost	311	276	256
Paid loss and loss expenses	1,071	914	921
Net written premiums	1,831	1,594	1,503
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	37	33	28
Reserves for unpaid claims and claim adjustment expenses	753	577	446
Unearned premiums	236	207	175
Earned premiums	485	398	325
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	324	243	192
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(9)	7	7
Amortization of deferred policy acquisition cost	79	63	51
Paid loss and loss expenses	145	118	112
Net written premiums	502	426	348
Reinsurance assumed and other non segment
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	105	79	61
Reserves for unpaid claims and claim adjustment expenses	1,099	831	611
Unearned premiums	437	356	273
Earned premiums	726	570	427
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	487	446	350
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(13)	(32)	(25)
Amortization of deferred policy acquisition cost	144	109	86
Paid loss and loss expenses	260	256	181
Net written premiums	$ 815	$ 648	$ 479