Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	$ 512	$ 464	$ 438
Reserves for unpaid claims and claim adjustment expenses	6,088	5,646	5,219
Unearned premiums	2,787	2,515	2,403
Earned premiums	5,334	4,920	4,722
Investment income	419	401	392
Loss and loss expenses incurred related to current accident year	3,600	3,390	3,257
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(248)	(167)	(119)
Amortization of deferred policy acquisition cost	986	907	867
Paid loss and loss expenses	3,260	2,847	2,843
Net written premiums	5,516	5,030	4,840
Commercial Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	311	291	284
Reserves for unpaid claims and claim adjustment expenses	4,569	4,466	4,236
Unearned premiums	1,665	1,576	1,548
Earned premiums	3,319	3,218	3,165
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,222	2,206	2,115
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(192)	(157)	(73)
Amortization of deferred policy acquisition cost	631	608	590
Paid loss and loss expenses	2,023	1,816	1,866
Net written premiums	3,410	3,245	3,202
Personal Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	130	126	121
Reserves for unpaid claims and claim adjustment expenses	687	679	587
Unearned premiums	757	725	683
Earned premiums	1,404	1,336	1,241
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,012	960	932
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(27)	13	(14)
Amortization of deferred policy acquisition cost	251	242	225
Paid loss and loss expenses	966	913	898
Net written premiums	1,435	1,378	1,294
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	25	21	17
Reserves for unpaid claims and claim adjustment expenses	351	298	264
Unearned premiums	152	123	105
Earned premiums	278	234	209
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	153	128	115
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(11)	(24)	(29)
Amortization of deferred policy acquisition cost	47	39	35
Paid loss and loss expenses	90	74	61
Net written premiums	303	249	219
Reinsurance assumed and other non segment
Property Casualty Insurance Underwriters
Deferred policy acquisition costs	46	26	16
Reserves for unpaid claims and claim adjustment expenses	481	203	132
Unearned premiums	213	91	67
Earned premiums	333	132	107
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	213	96	95
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(18)	1	(3)
Amortization of deferred policy acquisition cost	57	18	17
Paid loss and loss expenses	181	44	18
Net written premiums	$ 368	$ 158	$ 125