Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 464	$ 438	$ 408
Reserves for unpaid claims and claim adjustment expenses	5,646	5,219	5,035
Unearned premiums	2,515	2,403	2,306
Earned premiums	4,920	4,722	4,482
Investment income	401	392	384
Loss and loss expenses incurred related to current accident year	3,390	3,257	3,029
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(167)	(119)	(168)
Amortization of deferred policy acquisition cost	907	867	820
Paid loss and loss expenses	2,847	2,843	2,503
Net written premiums	5,030	4,840	4,580
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	291	284	271
Reserves for unpaid claims and claim adjustment expenses	4,466	4,236	4,179
Unearned premiums	1,576	1,548	1,510
Earned premiums	3,218	3,165	3,089
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,206	2,115	2,057
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(157)	(73)	(129)
Amortization of deferred policy acquisition cost	608	590	570
Paid loss and loss expenses	1,816	1,866	1,675
Net written premiums	3,245	3,202	3,122
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	126	121	110
Reserves for unpaid claims and claim adjustment expenses	679	587	569
Unearned premiums	725	683	629
Earned premiums	1,336	1,241	1,161
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	960	932	844
Prior year claims and claims adjustments expense, (favorable) and unfavorable	13	(14)	(4)
Amortization of deferred policy acquisition cost	242	225	209
Paid loss and loss expenses	913	898	771
Net written premiums	1,378	1,294	1,198
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	21	17	16
Reserves for unpaid claims and claim adjustment expenses	298	264	241
Unearned premiums	123	105	93
Earned premiums	234	209	183
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	128	115	102
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(24)	(29)	(34)
Amortization of deferred policy acquisition cost	39	35	31
Paid loss and loss expenses	74	61	55
Net written premiums	249	219	189
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	26	16	11
Reserves for unpaid claims and claim adjustment expenses	203	132	46
Unearned premiums	91	67	74
Earned premiums	132	107	49
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	96	95	26
Prior year claims and claims adjustments expense, (favorable) and unfavorable	1	(3)	(1)
Amortization of deferred policy acquisition cost	18	17	10
Paid loss and loss expenses	44	18	2
Net written premiums	$ 158	$ 125	$ 71