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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,128	$ 4,750	$ 5,035	$ 4,660
Less reinsurance recoverable	297	272	298	281
Net loss and loss expense reserves, beginning of period	4,831	4,478	4,737	4,379
Current accident year	832	808	1,658	1,531
Prior accident years	(38)	(49)	(76)	(111)
Total incurred	794	759	1,582	1,420
Current accident year	373	328	558	474
Prior accident years	322	301	831	717
Total paid	695	629	1,389	1,191
Net loss and loss expense reserves, end of period	4,930	4,608	4,930	4,608
Plus reinsurance recoverable	283	310	283	310
Gross loss and loss expense reserves, end of period	$ 5,213	$ 4,918	$ 5,213	$ 4,918