Supplementary Information Concerning Property Casualty Insurance Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 337		$ 303		$ 286
Reserves for unpaid claims and claim adjustment expenses	4,169		4,280		4,137
Unearned premiums	1,790		1,631		1,551
Earned premiums	3,344		3,029		2,924
Investment income	351	[1]	350	[1]	348	[1]
Loss and loss expenses incurred related to current accident year	2,533		2,620		2,319
Loss and loss expenses incurred related to prior accident year	(396)		(285)		(304)
Amortization of deferred policy acquisition cost	661		598		571
Paid loss and loss expenses	2,229		2,241		1,865
Net written premiums	3,482		3,098		2,963
Commercial Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	235		204		204
Reserves for unpaid claims and claim adjustment expenses	3,645		3,780		3,728
Unearned premiums	1,246		1,149		1,116
Earned premiums	2,383		2,197		2,154
Loss and loss expenses incurred related to current accident year	1,712		1,804		1,706
Loss and loss expenses incurred related to prior accident year	(292)		(234)		(269)
Amortization of deferred policy acquisition cost	461		441		423
Paid loss and loss expenses	1,546		1,545		1,330
Net written premiums	2,459		2,218		2,155
Personal Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	93		92		76
Reserves for unpaid claims and claim adjustment expenses	398		419		353
Unearned premiums	490		440		401
Earned premiums	868		762		721
Loss and loss expenses incurred related to current accident year	751		765		571
Loss and loss expenses incurred related to prior accident year	(99)		(42)		(34)
Amortization of deferred policy acquisition cost	183		144		139
Paid loss and loss expenses	659		676		526
Net written premiums	918		801		750
Excess And Surplus Lines Insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	9		7		6
Reserves for unpaid claims and claim adjustment expenses	126		81		56
Unearned premiums	54		42		34
Earned premiums	93		70		49
Loss and loss expenses incurred related to current accident year	70		51		42
Loss and loss expenses incurred related to prior accident year	(5)		(9)		(1)
Amortization of deferred policy acquisition cost	17		13		9
Paid loss and loss expenses	24		20		9
Net written premiums	$ 105		$ 79		$ 58

[1] This segment information is not regularly allocated to segments and not reviewed by company management in making decisions about resources to be allocated to the segments or to assess their performance.