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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,423	$ 5,213	$ 5,219	$ 5,035
Less reinsurance recoverable	188	283	187	298
Net loss and loss expense reserves, beginning of period	5,235	4,930	5,032	4,737
Current accident year	857	835	2,548	2,493
Prior accident years	(44)	(20)	(123)	(96)
Total incurred	813	815	2,425	2,397
Current accident year	392	411	928	969
Prior accident years	313	314	1,186	1,145
Total paid	705	725	2,114	2,114
Net loss and loss expense reserves, end of period	5,343	5,020	5,343	5,020
Plus reinsurance recoverable	186	280	186	280
Gross loss and loss expense reserves, end of period	$ 5,529	$ 5,300	$ 5,529	$ 5,300