Supplementary Information Concerning Property Casualty Insurance Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 366	$ 337	$ 303
Reserves for unpaid claims and claim adjustment expenses	4,241	4,169	4,280
Unearned premiums	1,970	1,790	1,631
Earned premiums	3,713	3,344	3,029
Investment income	348	351	350
Loss and loss expenses incurred related to current accident year	2,448	2,533	2,620
Loss and loss expenses incurred related to prior accident year	147	396	285
Amortization of deferred policy acquisition cost	727	661	598
Paid loss and loss expenses	2,172	2,229	2,241
Net written premiums	3,893	3,482	3,098
Commercial Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	251	235	204
Reserves for unpaid claims and claim adjustment expenses	3,667	3,645	3,780
Unearned premiums	1,370	1,246	1,149
Earned premiums	2,636	2,383	2,197
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	1,691	1,712	1,804
Loss and loss expenses incurred related to prior accident year	95	292	234
Amortization of deferred policy acquisition cost	514	461	441
Paid loss and loss expenses	1,498	1,546	1,545
Net written premiums	2,760	2,459	2,218
Personal Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	104	93	92
Reserves for unpaid claims and claim adjustment expenses	417	398	419
Unearned premiums	534	490	440
Earned premiums	961	868	762
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	678	751	765
Loss and loss expenses incurred related to prior accident year	39	99	42
Amortization of deferred policy acquisition cost	192	183	144
Paid loss and loss expenses	639	659	676
Net written premiums	1,005	918	801
Excess And Surplus Lines Insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	11	9	7
Reserves for unpaid claims and claim adjustment expenses	157	126	81
Unearned premiums	66	54	42
Earned premiums	116	93	70
Investment income	0	0	0
Loss and loss expenses incurred related to current accident year	79	70	51
Loss and loss expenses incurred related to prior accident year	13	5	9
Amortization of deferred policy acquisition cost	21	17	13
Paid loss and loss expenses	35	24	20
Net written premiums	$ 128	$ 105	$ 79