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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 6,880	$ 6,153	$ 6,677	$ 6,088
Less reinsurance recoverable	262	294	277	342
Net loss and loss expense reserves, beginning of period	6,618	5,859	6,400	5,746
Current accident year	949	1,054	1,982	2,017
Prior accident years	(119)	(47)	(229)	(80)
Total incurred	830	1,007	1,753	1,937
Current accident year	334	369	477	555
Prior accident years	433	382	995	1,013
Total paid	767	751	1,472	1,568
Net loss and loss expense reserves, end of period	6,681	6,115	6,681	6,115
Plus reinsurance recoverable	274	294	274	294
Gross loss and loss expense reserves, end of period	$ 6,955	$ 6,409	$ 6,955	$ 6,409