Supplementary Information Concerning Property Casualty Insurance Operations (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 330		$ 310		$ 303
Reserves for unpaid claims and claim adjustment expenses	4,280		4,137		4,096
Reserve discount deducted
Unearned premiums	1,629		1,551		1,507
Earned premiums	3,029		2,924		2,911
Investment income	350	[1]	348	[1]	336	[1]
Loss and loss expenses incurred related to current accident year	2,620		2,319		2,274
Loss and loss expenses incurred related to prior accident year	(285)		(304)		(188)
Amortization of deferred policy acquisition cost	649		616		611
Paid loss and loss expenses	2,241		1,865		1,923
Net written premiums	3,098		2,963		2,911
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	221		217		219
Reserves for unpaid claims and claim adjustment expenses	3,780		3,728		3,725
Unearned premiums	1,147		1,116		1,112
Earned premiums	2,197		2,154		2,199
Loss and loss expenses incurred related to current accident year	1,804		1,706		1,662
Loss and loss expenses incurred related to prior accident year	(234)		(269)		(147)
Amortization of deferred policy acquisition cost	473		454		458
Paid loss and loss expenses	1,545		1,330		1,348
Net written premiums	2,218		2,155		2,181
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	99		84		78
Reserves for unpaid claims and claim adjustment expenses	419		353		349
Unearned premiums	440		401		372
Earned premiums	762		721		685
Loss and loss expenses incurred related to current accident year	765		571		591
Loss and loss expenses incurred related to prior accident year	(42)		(34)		(40)
Amortization of deferred policy acquisition cost	157		148		143
Paid loss and loss expenses	676		526		573
Net written premiums	801		750		691
Excess And Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	10		9		6
Reserves for unpaid claims and claim adjustment expenses	81		56		22
Unearned premiums	42		34		23
Earned premiums	70		49		27
Loss and loss expenses incurred related to current accident year	51		42		21
Loss and loss expenses incurred related to prior accident year	(9)		(1)		(1)
Amortization of deferred policy acquisition cost	19		14		10
Paid loss and loss expenses	20		9		2
Net written premiums	$ 79		$ 58		$ 39

[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.