Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,219	$ 5,035
Less reinsurance recoverable	187	298
Net loss and loss expense reserves, beginning of period	5,032	4,737
Current accident year	839	826
Prior accident years	(48)	(38)
Total incurred	791	788
Current accident year	195	185
Prior accident years	519	509
Total paid	714	694
Net loss and loss expense reserves, end of period	5,109	4,831
Plus reinsurance recoverable	184	297
Gross loss and loss expense reserves, end of period	$ 5,293	$ 5,128