Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	$ 542	$ 512	$ 464
Reserves for unpaid claims and claim adjustment expenses	6,677	6,088	5,646
Unearned premiums	2,959	2,787	2,515
Earned premiums	5,691	5,334	4,920
Investment income	431	419	401
Loss and loss expenses incurred related to current accident year	3,968	3,600	3,390
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(131)	(248)	(167)
Amortization of deferred policy acquisition cost	1,057	986	907
Paid loss and loss expenses	3,183	3,260	2,847
Net written premiums	5,864	5,516	5,030
Commercial Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	317	311	291
Reserves for unpaid claims and claim adjustment expenses	4,881	4,569	4,466
Unearned premiums	1,714	1,665	1,576
Earned premiums	3,476	3,319	3,218
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,431	2,222	2,206
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(95)	(192)	(157)
Amortization of deferred policy acquisition cost	664	631	608
Paid loss and loss expenses	1,969	2,023	1,816
Net written premiums	3,534	3,410	3,245
Personal Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	136	130	126
Reserves for unpaid claims and claim adjustment expenses	739	687	679
Unearned premiums	797	757	725
Earned premiums	1,463	1,404	1,336
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	995	1,012	960
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(18)	(27)	13
Amortization of deferred policy acquisition cost	256	251	242
Paid loss and loss expenses	921	966	913
Net written premiums	1,503	1,435	1,378
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	28	25	21
Reserves for unpaid claims and claim adjustment expenses	446	351	298
Unearned premiums	175	152	123
Earned premiums	325	278	234
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	192	153	128
Prior year claims and claims adjustments expense, (favorable) and unfavorable	7	(11)	(24)
Amortization of deferred policy acquisition cost	51	47	39
Paid loss and loss expenses	112	90	74
Net written premiums	348	303	249
Reinsurance assumed and other non segment
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	61	46	26
Reserves for unpaid claims and claim adjustment expenses	611	481	203
Unearned premiums	273	213	91
Earned premiums	427	333	132
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	350	213	96
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(25)	(18)	1
Amortization of deferred policy acquisition cost	86	57	18
Paid loss and loss expenses	181	181	44
Net written premiums	$ 479	$ 368	$ 158