Supplementary Information Concerning Property Casualty Insurance Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 379	$ 366	$ 337
Reserves for unpaid claims and claim adjustment expenses	4,438	4,241	4,169
Unearned premiums	2,081	1,970	1,790
Earned premiums	4,045	3,713	3,344
Investment income	358	348	351
Loss and loss expenses incurred related to current accident year	2,725	2,448	2,533
Prior year claims and claims adjustment expense	(98)	(147)	(396)
Amortization of deferred policy acquisition cost	770	727	661
Paid loss and loss expenses	2,413	2,172	2,229
Net written premiums	4,143	3,893	3,482
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	257	251	235
Reserves for unpaid claims and claim adjustment expenses	3,797	3,667	3,645
Unearned premiums	1,441	1,370	1,246
Earned premiums	2,856	2,636	2,383
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,869	1,691	1,712
Prior year claims and claims adjustment expense	(57)	(95)	(292)
Amortization of deferred policy acquisition cost	537	514	461
Paid loss and loss expenses	1,666	1,498	1,546
Net written premiums	2,922	2,760	2,459
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	108	104	93
Reserves for unpaid claims and claim adjustment expenses	439	417	398
Unearned premiums	562	534	490
Earned premiums	1,041	961	868
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	752	678	751
Prior year claims and claims adjustment expense	(12)	(39)	(99)
Amortization of deferred policy acquisition cost	209	192	183
Paid loss and loss expenses	717	639	659
Net written premiums	1,068	1,005	918
Excess And Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	14	11	9
Reserves for unpaid claims and claim adjustment expenses	202	157	126
Unearned premiums	78	66	54
Earned premiums	148	116	93
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	104	79	70
Prior year claims and claims adjustment expense	(29)	(13)	(5)
Amortization of deferred policy acquisition cost	24	21	17
Paid loss and loss expenses	30	35	24
Net written premiums	$ 153	$ 128	$ 105