Segments Information - Revenues, Income Before Income Tax and Assets of Each Operating Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Revenues
Premiums earned related to insurance business											$ 11,834	$ 11,631		$ 11,202
Investment Income											1,556	1,644		1,665
Premiums earned related to reinsurance assumed											4	13		13
Net premiums earned											11,838	11,644		11,215
Corporate and other revenues											46	55		88
Realized investment gains, net											193	288		426
Total Revenues	3,400	[1]	3,362	3,423	3,410	3,345		3,420	3,400	3,420	13,595	13,585		13,319
Insurance underwriting income											498	475		1,162
Reinsurance assumed underwriting income											47	36		30
Underwriting income before deferred policy acquisition costs											545	511		1,192
Increase in deferred policy acquisition costs											3	63		30
Underwriting income											548	574		1,222
Investment income											1,518	1,605		1,630
Total property and casualty insurance											2,040	2,157		2,782
Corporate and other loss											(237)	(246)		(220)
Realized investment gains, net											193	288		426
INCOME BEFORE FEDERAL AND FOREIGN INCOME TAX											1,996	2,199		2,988
Assets
Total assets	52,184					50,445	[2]				52,184	50,445	[2]	49,976
Personal insurance [Member]
Revenues
Premiums earned related to insurance business											4,024	3,917		3,768
Insurance underwriting income											192	47		303
Commercial insurance [Member]
Revenues
Premiums earned related to insurance business											5,144	4,945		4,647
Insurance underwriting income											44	1		347
Specialty insurance [Member]
Revenues
Premiums earned related to insurance business											2,666	2,769		2,787
Insurance underwriting income											262	427		512
Property and casualty insurance [Member]
Revenues
Investment Income											1,518	1,598		1,590
Total Revenues											13,356	13,242		12,805
Investment income											1,482	1,562		1,558
Other income											10	21		2
Assets
Total assets	49,441					48,017					49,441	48,017		47,565
Corporate and Other [Member]
Assets
Total assets	2,830					2,513					2,830	2,513		2,483
Adjustments and eliminations [Member]
Assets
Total assets	$ (87)					$ (85)					$ (87)	$ (85)		$ (72)

[1] In the fourth quarter of 2012, revenues were reduced by net reinsurance reinstatement premium costs of $53 million and losses and expenses included net losses of $829 million related to Storm Sandy. Net income for the quarter was reduced by $573 million or $2.15 per diluted share ($2.16 per share basic) for the after-tax effect of the net costs. Excluding the impact of Storm Sandy, the losses to premiums earned ratio was 50.9%, the expenses to premiums written ratio was 30.4% and the combined ratio was 81.3%.

[2] As Adjusted