Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 4,750	$ 4,577	$ 4,660	$ 4,438
Less reinsurance recoverable	272	278	281	282
Net loss and loss expense reserves, beginning of period	4,478	4,299	4,379	4,156
Current accident year	808	724	1,531	1,435
Prior accident years	(49)	(70)	(111)	(92)
Total incurred	759	654	1,420	1,343
Current accident year	328	304	474	451
Prior accident years	301	294	717	693
Total paid	629	598	1,191	1,144
Net loss and loss expense reserves, end of period	4,608	4,355	4,608	4,355
Plus reinsurance recoverable	310	292	310	292
Gross loss and loss expense reserves, end of period	$ 4,918	$ 4,647	$ 4,918	$ 4,647