Consolidated Supplementary Insurance Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	$ 1,284	$ 1,255	$ 1,206
Unpaid Losses	22,678	23,146	23,963
Unearned Premiums	6,581	6,423	6,361
Premiums Earned	12,328	12,066	11,838
Net Investment Income	1,329	1,391	1,482
Insurance Losses	6,985	6,520	7,507
Amortization of Deferred Policy Acquisition Costs	2,548	2,454	2,411
Other Insurance Operating Costs and Expenses	1,393	1,417	1,372
Premiums Written	12,592	12,224	11,870
Personal Insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	517	505	477
Unpaid Losses	2,272	2,235	2,493
Unearned Premiums	2,278	2,216	2,133
Premiums Earned	4,418	4,214	4,024
Net Investment Income
Insurance Losses	2,540	2,224	2,417
Amortization of Deferred Policy Acquisition Costs	1,053	982	942
Other Insurance Operating Costs and Expenses	438	465	458
Premiums Written	4,508	4,322	4,125
Commercial Insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	504	493	477
Unpaid Losses	13,161	13,147	13,288
Unearned Premiums	2,876	2,792	2,794
Premiums Earned	5,281	5,237	5,144
Net Investment Income
Insurance Losses	3,111	2,891	3,512
Amortization of Deferred Policy Acquisition Costs	995	970	970
Other Insurance Operating Costs and Expenses	663	665	620
Premiums Written	5,402	5,273	5,174
Specialty Insurance [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	263	257	252
Unpaid Losses	6,847	7,307	7,622
Unearned Premiums	1,426	1,412	1,429
Premiums Earned	2,628	2,618	2,666
Net Investment Income
Insurance Losses	1,333	1,417	1,622
Amortization of Deferred Policy Acquisition Costs	497	502	499
Other Insurance Operating Costs and Expenses	295	287	293
Premiums Written	2,681	2,633	2,568
Reinsurance Assumed [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs
Unpaid Losses	398	457	560
Unearned Premiums	1	3	5
Premiums Earned	1	(3)	4
Net Investment Income
Insurance Losses	1	(12)	(44)
Amortization of Deferred Policy Acquisition Costs	3
Other Insurance Operating Costs and Expenses	(3)		1
Premiums Written	1	(4)	3
Investments [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs
Unpaid Losses
Unearned Premiums
Premiums Earned
Net Investment Income	1,329	1,391	1,482
Insurance Losses
Amortization of Deferred Policy Acquisition Costs
Other Insurance Operating Costs and Expenses
Premiums Written