Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,035	$ 4,660
Less reinsurance recoverable	298	281
Net loss and loss expense reserves, beginning of period	4,737	4,379
Current accident year	826	723
Prior accident years	(38)	(62)
Total incurred	788	661
Current accident year	185	146
Prior accident years	509	416
Total paid	694	562
Net loss and loss expense reserves, end of period	4,831	4,478
Plus reinsurance recoverable	297	272
Gross loss and loss expense reserves, end of period	$ 5,128	$ 4,750