Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	$ 602	$ 542	$ 512
Reserves for unpaid claims and claim adjustment expenses	7,229	6,677	6,088
Unearned premiums	3,270	2,959	2,787
Earned premiums	6,184	5,691	5,334
Investment income	457	431	419
Loss and loss expenses incurred related to current accident year	4,024	3,968	3,600
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(428)	(131)	(248)
Amortization of deferred policy acquisition cost	1,151	1,057	986
Paid loss and loss expenses	3,094	3,183	3,260
Net written premiums	6,479	5,864	5,516
Commercial Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	345	317	311
Reserves for unpaid claims and claim adjustment expenses	5,007	4,881	4,569
Unearned premiums	1,857	1,714	1,665
Earned premiums	3,674	3,476	3,319
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,293	2,431	2,222
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(353)	(95)	(192)
Amortization of deferred policy acquisition cost	703	664	631
Paid loss and loss expenses	1,806	1,969	2,023
Net written premiums	3,811	3,534	3,410
Personal Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	145	136	130
Reserves for unpaid claims and claim adjustment expenses	814	739	687
Unearned premiums	850	797	757
Earned premiums	1,542	1,463	1,404
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,042	995	1,012
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(50)	(18)	(27)
Amortization of deferred policy acquisition cost	276	256	251
Paid loss and loss expenses	914	921	966
Net written premiums	1,594	1,503	1,435
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	33	28	25
Reserves for unpaid claims and claim adjustment expenses	577	446	351
Unearned premiums	207	175	152
Earned premiums	398	325	278
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	243	192	153
Prior year claims and claims adjustments expense, (favorable) and unfavorable	7	7	(11)
Amortization of deferred policy acquisition cost	63	51	47
Paid loss and loss expenses	118	112	90
Net written premiums	426	348	303
Reinsurance assumed and other non segment
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	79	61	46
Reserves for unpaid claims and claim adjustment expenses	831	611	481
Unearned premiums	356	273	213
Earned premiums	570	427	333
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	446	350	213
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(32)	(25)	(18)
Amortization of deferred policy acquisition cost	109	86	57
Paid loss and loss expenses	256	181	181
Net written premiums	$ 648	$ 479	$ 368