Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	$ 749	$ 682	$ 602
Reserves for unpaid claims and claim adjustment expenses	8,975	8,336	7,229
Unearned premiums	4,118	3,688	3,270
Earned premiums	7,645	6,924	6,184
Investment income	602	509	457
Loss and loss expenses incurred related to current accident year	5,173	4,875	4,024
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(215)	(159)	(428)
Amortization of deferred policy acquisition cost	1,421	1,303	1,151
Paid loss and loss expenses	4,276	3,687	3,094
Net written premiums	8,046	7,307	6,479
Commercial Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	387	372	345
Reserves for unpaid claims and claim adjustment expenses	5,887	5,568	5,007
Unearned premiums	2,111	2,021	1,857
Earned premiums	4,264	4,024	3,674
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,910	2,837	2,293
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(123)	(76)	(353)
Amortization of deferred policy acquisition cost	808	769	703
Paid loss and loss expenses	2,480	2,211	1,806
Net written premiums	4,336	4,159	3,811
Personal Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	210	168	145
Reserves for unpaid claims and claim adjustment expenses	990	916	814
Unearned premiums	1,253	994	850
Earned premiums	2,044	1,689	1,542
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,506	1,227	1,042
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(64)	(61)	(50)
Amortization of deferred policy acquisition cost	367	311	276
Paid loss and loss expenses	1,353	1,071	914
Net written premiums	2,302	1,831	1,594
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	43	37	33
Reserves for unpaid claims and claim adjustment expenses	932	753	577
Unearned premiums	273	236	207
Earned premiums	542	485	398
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	361	324	243
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(11)	(9)	7
Amortization of deferred policy acquisition cost	89	79	63
Paid loss and loss expenses	183	145	118
Net written premiums	570	502	426
Reinsurance assumed and other non segment
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	109	105	79
Reserves for unpaid claims and claim adjustment expenses	1,166	1,099	831
Unearned premiums	481	437	356
Earned premiums	795	726	570
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	396	487	446
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(17)	(13)	(32)
Amortization of deferred policy acquisition cost	157	144	109
Paid loss and loss expenses	260	260	256
Net written premiums	$ 838	$ 815	$ 648