Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 408	$ 388	$ 379
Reserves for unpaid claims and claim adjustment expenses	5,035	4,660	4,438
Unearned premiums	2,306	2,200	2,081
Earned premiums	4,482	4,271	4,045
Investment income	384	368	358
Loss and loss expenses incurred related to current accident year	3,029	2,756	2,725
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(168)	(184)	(98)
Amortization of deferred policy acquisition cost	820	792	770
Paid loss and loss expenses	2,503	2,349	2,413
Net written premiums	4,580	4,361	4,143
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	271	264	257
Reserves for unpaid claims and claim adjustment expenses	4,179	3,925	3,797
Unearned premiums	1,510	1,472	1,441
Earned premiums	3,089	2,996	2,856
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	2,057	1,862	1,869
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(129)	(154)	(57)
Amortization of deferred policy acquisition cost	570	552	537
Paid loss and loss expenses	1,675	1,575	1,666
Net written premiums	3,122	3,025	2,922
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	110	103	108
Reserves for unpaid claims and claim adjustment expenses	569	498	439
Unearned premiums	629	593	562
Earned premiums	1,161	1,097	1,041
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	844	784	752
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(4)	5	(12)
Amortization of deferred policy acquisition cost	209	210	209
Paid loss and loss expenses	771	731	717
Net written premiums	1,198	1,128	1,068
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	16	15	14
Reserves for unpaid claims and claim adjustment expenses	241	227	202
Unearned premiums	93	87	78
Earned premiums	183	168	148
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	102	105	104
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(34)	(35)	(29)
Amortization of deferred policy acquisition cost	31	28	24
Paid loss and loss expenses	55	43	30
Net written premiums	189	175	153
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	11	6	0
Reserves for unpaid claims and claim adjustment expenses	46	10	0
Unearned premiums	74	48	0
Earned premiums	49	10	0
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	26	5	0
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(1)	0	0
Amortization of deferred policy acquisition cost	10	2	0
Paid loss and loss expenses	2	0	0
Net written premiums	$ 71	$ 33	$ 0