Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 4,438	$ 4,241
Less reinsurance recoverable	282	299
Net loss and loss expense reserves, beginning of period	4,156	3,942
Current accident year	711	705
Prior accident years	(22)	(29)
Total incurred	689	676
Current accident year	147	197
Prior accident years	399	387
Total paid	546	584
Net loss and loss expense reserves, end of period	4,299	4,034
Plus reinsurance recoverable	278	289
Gross loss and loss expense reserves, end of period	$ 4,577	$ 4,323