Consolidated Supplementary Insurance Information (Detail) (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	$ 1,206	[1]	$ 1,210	[1]	$ 1,142	[1]
Unpaid Losses	23,963		23,068		22,718
Unearned Premiums	6,361		6,322		6,189
Premiums Earned	11,838		11,644		11,215
Net Investment Income	1,482	[2]	1,562	[2]	1,558	[2]
Insurance Losses	7,507		7,407		6,499
Amortization of Deferred Policy Acquisition Costs	2,411	[1]	2,330	[1]	2,183	[1]
Other Insurance Operating Costs and Expenses	1,372	[1],[3]	1,333	[1],[3]	1,311	[1],[3]
Premiums Written	11,870		11,758		11,236
Personal insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	477	[1]	466	[1]	438	[1]
Unpaid Losses	2,493		2,241		2,144
Unearned Premiums	2,133		2,048		1,995
Premiums Earned	4,024		3,917		3,768
Net Investment Income		[2]		[2]		[2]
Insurance Losses	2,417		2,508		2,210
Amortization of Deferred Policy Acquisition Costs	942	[1]	900	[1]	836	[1]
Other Insurance Operating Costs and Expenses	458	[1],[3]	436	[1],[3]	397	[1],[3]
Premiums Written	4,125		3,977		3,825
Commercial insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	477	[1]	475	[1]	441	[1]
Unpaid Losses	13,288		12,422		11,807
Unearned Premiums	2,794		2,746		2,630
Premiums Earned	5,144		4,945		4,647
Net Investment Income		[2]		[2]		[2]
Insurance Losses	3,512		3,366		2,807
Amortization of Deferred Policy Acquisition Costs	970	[1]	929	[1]	866	[1]
Other Insurance Operating Costs and Expenses	620	[1],[3]	617	[1],[3]	621	[1],[3]
Premiums Written	5,174		5,051		4,676
Specialty insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	252	[1]	269	[1]	263	[1]
Unpaid Losses	7,622		7,677		7,872
Unearned Premiums	1,429		1,519		1,549
Premiums Earned	2,666		2,769		2,787
Net Investment Income		[2]		[2]		[2]
Insurance Losses	1,622		1,558		1,503
Amortization of Deferred Policy Acquisition Costs	499	[1]	501	[1]	481	[1]
Other Insurance Operating Costs and Expenses	293	[1],[3]	278	[1],[3]	289	[1],[3]
Premiums Written	2,568		2,720		2,727
Reinsurance Assumed [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs		[1]		[1]		[1]
Unpaid Losses	560		728		895
Unearned Premiums	5		9		15
Premiums Earned	4		13		13
Net Investment Income		[2]		[2]		[2]
Insurance Losses	(44)		(25)		(21)
Amortization of Deferred Policy Acquisition Costs		[1]		[1]		[1]
Other Insurance Operating Costs and Expenses	1	[1],[3]	2	[1],[3]	4	[1],[3]
Premiums Written	3		10		8
Investments [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs		[1]		[1]		[1]
Unpaid Losses
Unearned Premiums
Premiums Earned
Net Investment Income	1,482	[2]	1,562	[2]	1,558	[2]
Insurance Losses
Amortization of Deferred Policy Acquisition Costs		[1]		[1]		[1]
Other Insurance Operating Costs and Expenses		[1],[3]		[1],[3]		[1],[3]
Premiums Written

[1] Amounts in years prior to 2012 have been adjusted to reflect the adoption of new guidance issued by the Financial Accounting Standards Board related to the accounting for costs associated with acquiring or renewing insurance contracts. The adoption of this guidance decreased deferred policy acquisition costs by $420 million as of January 1, 2010. The effect of the adoption of the new guidance on net income was not material. Amortization of deferred policy acquisition costs and other insurance operating costs and expenses for the years ended December 31, 2011 and 2010 were retrospectively adjusted to conform to the change in accounting guidance. The amounts of the retrospective reductions to previously reported deferred policy acquisition costs as of December 31, 2011 and 2010 were the same as the amounts of the reductions as of January 1, 2010.

[2] Property and casualty assets are available for payment of losses and expenses for all classes of business; therefore, such assets and the related investment income have not been allocated to the underwriting segments.

[3] Other insurance operating costs and expenses does not include other income and charges.