Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 438	$ 408	$ 388
Reserves for unpaid claims and claim adjustment expenses	5,219	5,035	4,660
Unearned premiums	2,403	2,306	2,200
Earned premiums	4,722	4,482	4,271
Investment income	392	384	368
Loss and loss expenses incurred related to current accident year	3,257	3,029	2,756
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(119)	(168)	(184)
Amortization of deferred policy acquisition cost	867	820	792
Paid loss and loss expenses	2,843	2,503	2,349
Net written premiums	4,840	4,580	4,361
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	284	271	264
Reserves for unpaid claims and claim adjustment expenses	4,236	4,179	3,925
Unearned premiums	1,548	1,510	1,472
Earned premiums	3,165	3,089	2,996
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,115	2,057	1,862
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(73)	(129)	(154)
Amortization of deferred policy acquisition cost	590	570	552
Paid loss and loss expenses	1,866	1,675	1,575
Net written premiums	3,202	3,122	3,025
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	121	110	103
Reserves for unpaid claims and claim adjustment expenses	587	569	498
Unearned premiums	683	629	593
Earned premiums	1,241	1,161	1,097
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	932	844	784
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(14)	(4)	5
Amortization of deferred policy acquisition cost	225	209	210
Paid loss and loss expenses	898	771	731
Net written premiums	1,294	1,198	1,128
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	17	16	15
Reserves for unpaid claims and claim adjustment expenses	264	241	227
Unearned premiums	105	93	87
Earned premiums	209	183	168
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	115	102	105
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(29)	(34)	(35)
Amortization of deferred policy acquisition cost	35	31	28
Paid loss and loss expenses	61	55	43
Net written premiums	219	189	175
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	16	11	6
Reserves for unpaid claims and claim adjustment expenses	132	46	10
Unearned premiums	67	74	48
Earned premiums	107	49	10
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	95	26	5
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(3)	(1)	0
Amortization of deferred policy acquisition cost	17	10	2
Paid loss and loss expenses	18	2	0
Net written premiums	$ 125	$ 71	$ 33