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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,213	$ 4,918	$ 5,035	$ 4,660
Less reinsurance recoverable	283	310	298	281
Net loss and loss expense reserves, beginning of period	4,930	4,608	4,737	4,379
Current accident year	835	730	2,493	2,261
Prior accident years	(20)	(40)	(96)	(151)
Total incurred	815	690	2,397	2,110
Current accident year	411	374	969	848
Prior accident years	314	288	1,145	1,005
Total paid	725	662	2,114	1,853
Net loss and loss expense reserves, end of period	5,020	4,636	5,020	4,636
Plus reinsurance recoverable	280	301	280	301
Gross loss and loss expense reserves, end of period	$ 5,300	$ 4,937	$ 5,300	$ 4,937