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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,293	$ 5,128	$ 5,219	$ 5,035
Less reinsurance recoverable	184	297	187	298
Net loss and loss expense reserves, beginning of period	5,109	4,831	5,032	4,737
Current accident year	852	832	1,691	1,658
Prior accident years	(31)	(38)	(79)	(76)
Total incurred	821	794	1,612	1,582
Current accident year	341	373	536	558
Prior accident years	354	322	873	831
Total paid	695	695	1,409	1,389
Net loss and loss expense reserves, end of period	5,235	4,930	5,235	4,930
Plus reinsurance recoverable	188	283	188	283
Gross loss and loss expense reserves, end of period	$ 5,423	$ 5,213	$ 5,423	$ 5,213