Consolidated Property Casualty Loss and Loss Expense Reserves (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]
Gross loss and loss expense reserves, beginning of period	$ 4,137	$ 4,096	$ 4,040
Less reinsurance receivable	326	435	542
Net loss and loss expense reserves, beginning of period	3,811	3,661	3,498
Current accident year	2,620	2,319	2,274
Prior accident years	(285)	(304)	(188)
Total incurred	2,335	2,015	2,086
Current accident year	1,206	939	929
Prior accident years	1,035	926	994
Total paid	2,241	1,865	1,923
Net loss and loss expense reserves, end of period	3,905	3,811	3,661
Plus reinsurance receivable	375	326	435
Gross loss and loss expense reserves, end of period	$ 4,280	$ 4,137	$ 4,096