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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 4,647	$ 4,408	$ 4,438	$ 4,241
Less reinsurance recoverable	292	282	282	299
Net loss and loss expense reserves, beginning of period	4,355	4,126	4,156	3,942
Current accident year	661	647	2,096	2,125
Prior accident years	(48)	(25)	(140)	(120)
Total incurred	613	622	1,956	2,005
Current accident year	343	353	794	859
Prior accident years	261	286	954	979
Total paid	604	639	1,748	1,838
Net loss and loss expense reserves, end of period	4,364	4,109	4,364	4,109
Plus reinsurance recoverable	289	284	289	284
Gross loss and loss expense reserves, end of period	$ 4,653	$ 4,393	$ 4,653	$ 4,393