Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 4,918	$ 4,647	$ 4,660	$ 4,438
Less reinsurance recoverable	310	292	281	282
Net loss and loss expense reserves, beginning of period	4,608	4,355	4,379	4,156
Current accident year	730	661	2,261	2,096
Prior accident years	(40)	(48)	(151)	(140)
Total incurred	690	613	2,110	1,956
Current accident year	374	343	848	794
Prior accident years	288	261	1,005	954
Total paid	662	604	1,853	1,748
Net loss and loss expense reserves, end of period	4,636	4,364	4,636	4,364
Plus reinsurance recoverable	301	289	301	289
Gross loss and loss expense reserves, end of period	$ 4,937	$ 4,653	$ 4,937	$ 4,653