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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 6,955	$ 6,409	$ 6,677	$ 6,088
Less reinsurance recoverable	274	294	277	342
Net loss and loss expense reserves, beginning of period	6,681	6,115	6,400	5,746
Current accident year	1,090	1,082	3,072	3,099
Prior accident years	(102)	(11)	(331)	(91)
Total incurred	988	1,071	2,741	3,008
Current accident year	416	443	893	998
Prior accident years	349	336	1,344	1,349
Total paid	765	779	2,237	2,347
Net loss and loss expense reserves, end of period	6,904	6,407	6,904	6,407
Plus reinsurance recoverable	322	285	322	285
Gross loss and loss expense reserves, end of period	$ 7,226	$ 6,692	$ 7,226	$ 6,692