Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 388	$ 379	$ 366
Reserves for unpaid claims and claim adjustment expenses	4,660	4,438	4,241
Unearned premiums	2,200	2,081	1,970
Earned premiums	4,271	4,045	3,713
Investment income	368	358	348
Loss and loss expenses incurred related to current accident year	2,756	2,725	2,448
Prior year claims and claims adjustment expense	(184)	(98)	(147)
Amortization of deferred policy acquisition cost	792	770	727
Paid loss and loss expenses	2,349	2,413	2,172
Net written premiums	4,361	4,143	3,893
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	264	257	251
Reserves for unpaid claims and claim adjustment expenses	3,925	3,797	3,667
Unearned premiums	1,472	1,441	1,370
Earned premiums	2,996	2,856	2,636
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,862	1,869	1,691
Prior year claims and claims adjustment expense	(154)	(57)	(95)
Amortization of deferred policy acquisition cost	552	537	514
Paid loss and loss expenses	1,575	1,666	1,498
Net written premiums	3,025	2,922	2,760
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	103	108	104
Reserves for unpaid claims and claim adjustment expenses	498	439	417
Unearned premiums	593	562	534
Earned premiums	1,097	1,041	961
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	784	752	678
Prior year claims and claims adjustment expense	5	(12)	(39)
Amortization of deferred policy acquisition cost	210	209	192
Paid loss and loss expenses	731	717	639
Net written premiums	1,128	1,068	1,005
Excess And Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	15	14	11
Reserves for unpaid claims and claim adjustment expenses	227	202	157
Unearned premiums	87	78	66
Earned premiums	168	148	116
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	105	104	79
Prior year claims and claims adjustment expense	(35)	(29)	(13)
Amortization of deferred policy acquisition cost	28	24	21
Paid loss and loss expenses	43	30	35
Net written premiums	175	153	128
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	6	0	0
Reserves for unpaid claims and claim adjustment expenses	10	0	0
Unearned premiums	48	0	0
Earned premiums	10	0	0
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	5	0	0
Prior year claims and claims adjustment expense	0	0	0
Amortization of deferred policy acquisition cost	2	0	0
Paid loss and loss expenses	0	0	0
Net written premiums	$ 33	$ 0	$ 0