Supplementary Insurance Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 510		$ 488		$ 481
Gross future policy benefits, losses, claims and expense losses	6,537	[1]	6,210	[1]	5,913	[1]
Gross unearned premium	1,633	[1]	1,553	[1]	1,509	[1]
Other policy claims and benefits payable	16	[1]	24	[1]	12	[1]
Earned premiums	3,194		3,082		3,054
Investment income, net of expenses	484		477		458
Benefits, claims losses and settlement expense	2,524		2,180		2,242
Amortization of deferred policy acquisition costs	685	[2]	653	[2]	638	[2]
Other underwriting and insurance expenses	351	[2]	368	[2]	366	[2]
Net written premiums	3,101		2,966		2,914
Property, Liability and Casualty Insurance Segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	330		310		303
Gross future policy benefits, losses, claims and expense losses	4,280		4,137		4,096
Gross unearned premium	1,631		1,551		1,507
Earned premiums	3,029		2,924		2,911
Investment income, net of expenses	350	[3]	348	[3]	336	[3]
Benefits, claims losses and settlement expense	2,335		2,015		2,086
Amortization of deferred policy acquisition costs	649		616		611
Other underwriting and insurance expenses	325		344		343
Net written premiums	3,098		2,963		2,911
Property, Liability and Casualty Insurance Segment | Commercial Lines
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	221		217		219
Gross future policy benefits, losses, claims and expense losses	3,780		3,728		3,725
Gross unearned premium	1,149		1,116		1,112
Earned premiums	2,197		2,154		2,199
Benefits, claims losses and settlement expense	1,570		1,437		1,515
Amortization of deferred policy acquisition costs	473		454		458
Other underwriting and insurance expenses	259		250		261
Net written premiums	2,218		2,155		2,181
Property, Liability and Casualty Insurance Segment | Personal Lines
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	99		84		78
Gross future policy benefits, losses, claims and expense losses	419		353		349
Gross unearned premium	440		401		372
Earned premiums	762		721		685
Benefits, claims losses and settlement expense	723		537		551
Amortization of deferred policy acquisition costs	157		148		143
Other underwriting and insurance expenses	64		92		71
Net written premiums	801		750		691
Property, Liability and Casualty Insurance Segment | Excess And Surplus Lines Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	10		9		6
Gross future policy benefits, losses, claims and expense losses	81		56		22
Gross unearned premium	42		34		23
Earned premiums	70		49		27
Benefits, claims losses and settlement expense	42		41		20
Amortization of deferred policy acquisition costs	19		14		10
Other underwriting and insurance expenses	2		2		11
Net written premiums	79		58		39
Life Insurance Segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	180		178		178
Gross future policy benefits, losses, claims and expense losses	2,257		2,073		1,817
Gross unearned premium	2		2		2
Other policy claims and benefits payable	16		24		12
Earned premiums	165		158		143
Investment income, net of expenses	134		129		122
Benefits, claims losses and settlement expense	189		170		160
Amortization of deferred policy acquisition costs	36		37		27
Other underwriting and insurance expenses	26		24		23
Net written premiums	3		3		3
Consolidation, Eliminations
Supplementary Insurance Information, by Segment [Line Items]
Benefits, claims losses and settlement expense			$ (5)		$ (4)

[1] The sum of gross future policy benefits, losses, claims and expense losses, gross unearned premium and other policy claims and benefits payable is equal to the sum of Loss and loss expense reserves, Life policy reserves and Unearned premiums reported in the company's consolidated balance sheets, Page 110.

[2] The sum of amortization of deferred policy acquisition costs and other underwriting and insurance expenses is equal to underwriting, acquisition and insurance expenses in the consolidated statements of income.

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